[cmake] Don't build Native llvm-config when cross compiling if passed by user.

[hintonda]

Summary

Rename LLVM_CONFIG_EXE to LLVM_CONFIG_PATH, and avoid building it if
passed in by user. This is the same way CLANG_TABLEGEN and
LLVM_TABLEGEN are handled, e.g., when -DLLVM_OPTIMIZED_TABLEGEN=ON is
passed.

[hintonda]

• Don't build Native LLVM_CONFIG_EXE when cross compiling if passed by

[phosek]

I think LLVM_CONFIG_PATH is a more appropriate as that name is already used by other LLVM projects exactly for this purpose.

[hintonda]

• Change name to LLVM_CONFIG_PATH per suggestion by @phosek.

[martell]

This seems to work better then what I have in D41580 for llvm_config.
LGTM.

We should try to get this back ported to the 6.0 branch before release.

[hintonda]

Add LLVM_CONFIG_PATH to the cache instead of using PARENT_SCOPE, since
PARENT_SCOPE only goes up one level -- tools directory in this case --
and won't be visible to llvm_ExternalProject_Add otherwise.

llvm/trunk/cmake/modules/LLVMExternalProjectUtils.cmake

if(CMAKE_SYSROOT)
     set(sysroot_arg -DCMAKE_SYSROOT=${CMAKE_SYSROOT})
   endif()
 
   if(CMAKE_CROSSCOMPILING)
     set(compiler_args -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
                       -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
                       -DCMAKE_AR=${CMAKE_AR}
                       -DCMAKE_RANLIB=${CMAKE_RANLIB})
-    set(llvm_config_path "${LLVM_NATIVE_BUILD}/bin/llvm-config")
+    set(llvm_config_path ${LLVM_CONFIG_PATH})
   else()
     set(llvm_config_path "$<TARGET_FILE:llvm-config>")
   endif()
 
   ExternalProject_Add(${name}
     DEPENDS ${ARG_DEPENDS} llvm-config
     ${name}-clobber
     PREFIX ${CMAKE_BINARY_DIR}/projects/${name}

llvm/trunk/tools/llvm-config/CMakeLists.txt

if(LLVM_ENABLE_MODULES)
   target_compile_options(llvm-config PUBLIC
     "-fmodules-ignore-macro=CMAKE_CFG_INTDIR"
     )
 endif()
 
 # Add the dependency on the generation step.
 add_file_dependencies(${CMAKE_CURRENT_SOURCE_DIR}/llvm-config.cpp ${BUILDVARIABLES_OBJPATH})
 
-if(CMAKE_CROSSCOMPILING)
-  set(${project}_LLVM_CONFIG_EXE "${LLVM_NATIVE_BUILD}/bin/llvm-config")
-  set(${project}_LLVM_CONFIG_EXE ${${project}_LLVM_CONFIG_EXE} PARENT_SCOPE)
+if(CMAKE_CROSSCOMPILING AND NOT LLVM_CONFIG_PATH)
+  set(LLVM_CONFIG_PATH "${LLVM_NATIVE_BUILD}/bin/llvm-config" CACHE STRING "")
 
-  add_custom_command(OUTPUT "${${project}_LLVM_CONFIG_EXE}"
+  add_custom_command(OUTPUT "${LLVM_CONFIG_PATH}"
     COMMAND ${CMAKE_COMMAND} --build . --target llvm-config --config $<CONFIGURATION>
     DEPENDS ${LLVM_NATIVE_BUILD}/CMakeCache.txt
     WORKING_DIRECTORY ${LLVM_NATIVE_BUILD}
     COMMENT "Building native llvm-config..."
     USES_TERMINAL)
-  add_custom_target(${project}NativeLLVMConfig DEPENDS ${${project}_LLVM_CONFIG_EXE})
-  add_dependencies(${project}NativeLLVMConfig CONFIGURE_LLVM_NATIVE)
+  add_custom_target(NativeLLVMConfig DEPENDS ${LLVM_CONFIG_PATH})
+  add_dependencies(NativeLLVMConfig CONFIGURE_LLVM_NATIVE)
 
-  add_dependencies(llvm-config ${project}NativeLLVMConfig)
-endif(CMAKE_CROSSCOMPILING)
+  add_dependencies(llvm-config NativeLLVMConfig)
+endif()