fail build when Tcl can't be found

But only if the user asks for Tcl specifically. We should probably have similar
logic for the other SML languages, but Tcl is especially troublesome.

.github/workflows/build.yml

@@ -49,7 +49,7 @@ jobs:
         env:
           # instructs SCons to print debug statements to stdout
           SCONS_MSCOMMON_DEBUG: '-'
-        run: python3 scons/scons.py --scu --opt --verbose all performance_tests
+        run: python3 scons/scons.py --scu --opt --verbose all performance_tests sml_tcl
 
       - name: Pack artifacts
         run: tar -czvf build.tar.gz out/
@@ -169,7 +169,7 @@ jobs:
           # instructs SCons to print debug statements to stdout
           SCONS_MSCOMMON_DEBUG: '-'
         run: |
-          ./build.bat --scu --opt --verbose --tcl="$env:TCL_HOME" all performance_tests
+          ./build.bat --scu --opt --verbose --tcl="$env:TCL_HOME" all performance_tests sml_tcl
           if ($lastexitcode -ne 0) {
              throw "build.bat exit code: $lastexitcode"
           }

Core/ClientSMLSWIG/Tcl/SConscript

@@ -18,6 +18,8 @@ LIB_NAME = 'Tcl_sml_ClientInterface'
 
 if clone.PrepareForCompilingWithTcl(clone["TCL_PATH"], clone["TCL_SUFFIX"]):
     print(f'{env["INDENT"]}Tcl SML library is buildable')
+elif clone['TCL_REQUIRED']:
+    raise ValueError('Tcl was requested by user explicitly, but Tcl is not available')
 else:
     print(f'{env["INDENT"]}Tcl SML library is *not* buildable')
     Return()

SConstruct

@@ -152,6 +152,10 @@ env = Environment(
     OUT_DIR=os.path.realpath(GetOption('outdir')),
     TCL_PATH = GetOption('tcl'),
     TCL_SUFFIX = GetOption('tcl_suffix'),
+    # We fail the build immediately if Tcl cannot be loaded but was specifically requested.
+    # This is done because the Tcl build is super fragile and it's easy to accidentally
+    # build without it.
+    TCL_REQUIRED = 'tclsoarlib' in COMMAND_LINE_TARGETS or 'sml_tcl' in COMMAND_LINE_TARGETS,
     SOAR_VERSION=SOAR_VERSION,
     VISHIDDEN=False,  # needed by swig
 	JAVAVERSION='11.0',

Tcl/SConscript

@@ -13,6 +13,8 @@ LIB_NAME = 'tclsoarlib'
 
 if clone.PrepareForCompilingWithTcl(clone["TCL_PATH"], clone["TCL_SUFFIX"]):
     print(f'{env["INDENT"]}TclSoarLib is buildable')
+elif clone['TCL_REQUIRED']:
+    raise ValueError('Tcl was requested by user explicitly, but Tcl is not available')
 else:
     print(f'{env["INDENT"]}TclSoarLib is *not* buildable')
     Return()