Set CC and CXX environment variables for GCC/Clang toolchain

Sets the C (CC) and C++ (CXX) compiler environment variables so the
choice of toolchain selected in the Build Settings tab is respected.

Toolchains supported are GNU GCC and Clang.

Cross prefix is supported for GNU GCC. Version suffix and extension
supported.

Added JUnit tests for GCCToolChain and ClangToolChain.
Added test document describing manual test steps.

Fixes #1140

Author: betamaxbandit

build/org.eclipse.cdt.build.gcc.core.tests/META-INF/MANIFEST.MF

@@ -9,7 +9,8 @@ Require-Bundle: org.eclipse.core.resources,
  org.junit,
  org.eclipse.jdt.junit4.runtime,
  org.eclipse.core.runtime,
- org.eclipse.cdt.build.gcc.core
+ org.eclipse.cdt.build.gcc.core,
+ junit-jupiter-api
 Automatic-Module-Name: org.eclipse.cdt.build.gcc.core.tests
 Bundle-RequiredExecutionEnvironment: JavaSE-17
 Bundle-ActivationPolicy: lazy

build/org.eclipse.cdt.build.gcc.core.tests/manualTests/README.md

@@ -0,0 +1,133 @@
+## Overview
+
+This document captures the manual tests that should be completed for Core Build toolchains.
+
+Added:
+
+  20251010 John Moule
+  Set CC and CXX environment variables for clang toolchain #1169
+  https://github.com/eclipse-cdt/cdt/pull/1169
+  https://github.com/eclipse-cdt/cdt/issues/1140
+
+## Tests
+The manual test cases below are useful to show the expected toolchain is used for a build.
+
+The following JUnits are also added:
+-  org.eclipse.cdt.build.gcc.core.tests.TestGCCToolChain
+-  org.eclipse.cdt.build.gcc.core.tests.TestClangToolChain
+
+## Test Case Summary
+- A) Clang toolchain can be used to build when selected in Build Settings > Toolchain.
+- B) GCC toolchain can be used to build when selected in Build Settings > Toolchain.
+- C) Clang toolchain can be used to build when CC/CXX defined externally to gcc/g++
+- D) GCC toolchain can be used to build when CC/CXX defined externally to clang/clang++
+
+Tests should be performed on Linux & Windows.
+
+### Prerequisites
+- Make sure cmake, make, ninja, GCC and clang toolchains are installed on the host - see [1].
+- Make sure the environment variables CC/CXX are not defined in the system environment, unless specified in the test.
+
+
+## Test case A) Clang toolchain can be used to build when selected in Build Settings > Toolchain.
+- Create a CMake Hello World project
+- From the Launch Bar, open the Launch Configuration (gear icon)
+- Select Build Settings tab and in the Toolchain dropdown, change to clang toolchain and click OK.
+- From the Launch Bar, build the project.
+
+Expected: The project builds successfully and in the Console build output, it can be seen that the C and CXX compiler are identified as "Clang".
+
+Actual: The following shows an example of Console build output:
+
+
+    Configuring in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
+    cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw
+    -- The C compiler identification is Clang 19.1.7
+    -- The CXX compiler identification is Clang 19.1.7
+    -- Detecting C compiler ABI info
+    -- Detecting C compiler ABI info - done
+    -- Check for working C compiler: C:/msys64/ucrt64/bin/clang.exe - skipped
+    -- Detecting C compile features
+    -- Detecting C compile features - done
+    -- Detecting CXX compiler ABI info
+    -- Detecting CXX compiler ABI info - done
+    -- Check for working CXX compiler: C:/msys64/ucrt64/bin/clang++.exe - skipped
+    -- Detecting CXX compile features
+    -- Detecting CXX compile features - done
+    -- Configuring done (3.1s)
+    -- Generating done (0.1s)
+    -- Build files have been written to: C:/Users/a5107948/cdt-main20250110b/runtime-cdtruntime_clean/cmake_hw/build/cmake.debug.win32.x86_64.Local
+    Building in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
+    cmake --build . --target all
+    [1/2] Building CXX object CMakeFiles/cmake_hw.dir/cmake_hw.cpp.obj
+    [2/2] Linking CXX executable cmake_hw.exe
+    Build complete (0 errors, 0 warnings): C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
+
+
+## Test case B) GCC toolchain can be used to build when selected in Build Settings > Toolchain.
+- From the Launch Bar, open the Launch Configuration (gear icon)
+- Select Build Settings tab and in the Toolchain dropdown, change to gcc toolchain and click OK.
+- Delete the project's build folder.
+- From the Launch Bar, build the project.
+
+Expected: The project builds successfully and in the Console build output, it can be seen that the C and CXX compiler are identified as "gnu".
+
+Actual: The following shows an example of Console build output:
+
+    Configuring in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
+    cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw
+    -- The C compiler identification is GNU 14.2.0
+    -- The CXX compiler identification is GNU 14.2.0
+    -- Detecting C compiler ABI info
+    -- Detecting C compiler ABI info - done
+    -- Check for working C compiler: C:/msys64/ucrt64/bin/gcc.exe - skipped
+    -- Detecting C compile features
+    -- Detecting C compile features - done
+    -- Detecting CXX compiler ABI info
+    -- Detecting CXX compiler ABI info - done
+    -- Check for working CXX compiler: C:/msys64/ucrt64/bin/g++.exe - skipped
+    -- Detecting CXX compile features
+    -- Detecting CXX compile features - done
+    -- Configuring done (3.4s)
+    -- Generating done (0.1s)
+    -- Build files have been written to: C:/Users/a5107948/cdt-main20250110b/runtime-cdtruntime_clean/cmake_hw/build/cmake.debug.win32.x86_64.Local
+    Building in: C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
+    cmake --build . --target all
+    [1/2] Building CXX object CMakeFiles/cmake_hw.dir/cmake_hw.cpp.obj
+    [2/2] Linking CXX executable cmake_hw.exe
+    Build complete (0 errors, 0 warnings): C:\Users\a5107948\cdt-main20250110b\runtime-cdtruntime_clean\cmake_hw\build\cmake.debug.win32.x86_64.Local
+
+
+
+## Test case C) Clang toolchain can be used to build when CC/CXX defined externally to gcc/g++
+- Exit Eclipse and in a command shell, define the environment variables CC and CXX, for example on Windows:
+- - set CC=gcc
+- - set CXX=g++
+- Launch Eclipse from this command shell, so it inherits the environment.
+- From the Launch Bar, open the Launch Configuration (gear icon)
+- Select Build Settings tab and in the Toolchain dropdown, change to clang toolchain and click OK.
+- From the Launch Bar, build the project.
+
+Expected: The project builds successfully and in the Console build output, it can be seen that the C and CXX compiler are identified as "Clang".
+
+Actual: same output as Test case A).
+
+## Test case D) GCC toolchain can be used to build when CC/CXX defined externally to clang/clang++
+- Exit Eclipse and in a command shell, define the environment variables CC and CXX, for example on Windows:
+- - set CC=clang
+- - set CXX=clang++
+- Launch Eclipse from this command shell, so it inherits the environment.
+- From the Launch Bar, open the Launch Configuration (gear icon)
+- Select Build Settings tab and in the Toolchain dropdown, change to gcc toolchain and click OK.
+- From the Launch Bar, build the project.
+
+Expected: The project builds successfully and in the Console build output, it can be seen that the C and CXX compiler are identified as "gnu".
+
+Actual: same output as Test case B).
+
+----
+[1]: Before you begin
+C/C++ Development User Guide > Before you begin
+https://github.com/eclipse-cdt/cdt/blob/abe4d1b8e16b03ceafbeb25f063cb728c7487862/doc/org.eclipse.cdt.doc.user/concepts/cdt_c_before_you_begin.htm
+
+