add CODE_GENERATOR option for cxx-build

This sets the GENERATED_HEADER_DIR environment variable when calling
cargo and adds that path to the target's include paths. Cargo build
scripts can read this to determine where to output generated C/C++
headers.

Depends on dtolnay/cxx#1120 for cxx-build

Fixes corrosion-rs#204

Author: Be-ing

README.md

@@ -192,6 +192,8 @@ corrosion_import_crate(MANIFEST_PATH <path/to/cargo.toml>
         [NO_DEFAULT_FEATURES]
         # Disable linking of standard libraries (required for no_std crates).
         [NO_STD]
+        # Add headers generated by cxx-build or cxx-qt-build to include paths
+        [CODE_GENERATOR]
         # Specify  cargo build profile (e.g. release or a custom profile)
         [PROFILE <cargo-profile>]
         # Only import the specified crates from a workspace

cmake/Corrosion.cmake

@@ -821,6 +821,11 @@ function(_add_cargo_build out_cargo_build_out_dir)
     if(COR_NO_STD)
         set(no_default_libraries_arg --no-default-libraries)
     endif()
+    if(COR_CODE_GENERATOR)
+        set(generated_header_dir "${CMAKE_CURRENT_BINARY_DIR}/generated-code")
+        target_include_directories("${target_name}" INTERFACE "${generated_header_dir}")
+        set(generated_header_dir_env "GENERATED_HEADER_DIR=${generated_header_dir}")
+    endif()
 
     set(global_rustflags_target_property "$<TARGET_GENEX_EVAL:${target_name},$<TARGET_PROPERTY:${target_name},INTERFACE_CORROSION_RUSTFLAGS>>")
     set(local_rustflags_target_property  "$<TARGET_GENEX_EVAL:${target_name},$<TARGET_PROPERTY:${target_name},INTERFACE_CORROSION_LOCAL_RUSTFLAGS>>")
@@ -954,6 +959,7 @@ function(_add_cargo_build out_cargo_build_out_dir)
                 "${cargo_target_linker}"
                 "${corrosion_cc_rs_flags}"
                 "${cargo_library_path}"
+                "${generated_header_dir_env}"
                 "CORROSION_BUILD_DIR=${CMAKE_CURRENT_BINARY_DIR}"
                 "CARGO_BUILD_RUSTC=${_CORROSION_RUSTC}"
             "${_CORROSION_CARGO}"
@@ -1026,7 +1032,7 @@ function(_add_cargo_build out_cargo_build_out_dir)
 endfunction()
 
 function(corrosion_import_crate)
-    set(OPTIONS ALL_FEATURES NO_DEFAULT_FEATURES NO_STD)
+    set(OPTIONS ALL_FEATURES NO_DEFAULT_FEATURES NO_STD CODE_GENERATOR)
     set(ONE_VALUE_KEYWORDS MANIFEST_PATH PROFILE)
     set(MULTI_VALUE_KEYWORDS CRATES FEATURES FLAGS)
     cmake_parse_arguments(COR "${OPTIONS}" "${ONE_VALUE_KEYWORDS}" "${MULTI_VALUE_KEYWORDS}" ${ARGN})