[OpenMP] Fix an OpenMP Windows build problem

[vadikp-intel]

Summary

When built as part of LLVM, CMAKE is generating incorrect locations references for the first build step's artifacts being used in regenerating the Windows import library in the second step. The fix is to feed a dummy non-buildable, rather than buildable, source to CMAKE to satisfy its source requirement in the second step removing the need to reference the first step's artifacts.

[vadikp-intel]

Removed redundant comments.

[vadikp-intel]

Updated with a better fix.

[mstorsjo]

LGTM, thanks! This is indeed the neatest solution so far!

openmp/runtime/src/CMakeLists.txt

302

Can't we get rid of the omp dependency here at this point too, since there's no technical need to build it after the main library has been built?

[vadikp-intel]

removed a now redundant dependency

openmp/runtime/src/CMakeLists.txt

if(MSVC)
     set(LIBOMPIMP_GENERATED_DEF_FILE ${LIBOMP_LIB_NAME}.imp.def)
     add_custom_target(libompimp-needed-def-file DEPENDS ${LIBOMPIMP_GENERATED_DEF_FILE})
     add_custom_command(
       OUTPUT  ${LIBOMPIMP_GENERATED_DEF_FILE}
       COMMAND ${PERL_EXECUTABLE} ${LIBOMP_TOOLS_DIR}/generate-def.pl ${LIBOMP_GDFLAGS} -D NAME=${LIBOMP_LIB_FILE} -D NOORDINALS
           -o ${LIBOMPIMP_GENERATED_DEF_FILE} ${CMAKE_CURRENT_SOURCE_DIR}/dllexports
       DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/dllexports ${LIBOMP_TOOLS_DIR}/generate-def.pl
     )
-    # while this is merely generating an import library off the one generated with the runtime dll,
-    # kmp_global.cpp will satisfy the librarian's apparent ask to see the actual exported DATA items
-    set_source_files_properties(${LIBOMP_GENERATED_IMP_LIB_FILENAME} PROPERTIES GENERATED TRUE EXTERNAL_OBJECT TRUE)
-    add_library(${LIBOMP_IMP_LIB_TARGET} STATIC ${LIBOMP_GENERATED_IMP_LIB_FILENAME} kmp_global.cpp)
+    add_library(${LIBOMP_IMP_LIB_TARGET} STATIC ${LIBOMP_SOURCE_FILES})
     set_target_properties(${LIBOMP_IMP_LIB_TARGET} PROPERTIES
-        PREFIX "" SUFFIX "" OUTPUT_NAME "${LIBOMP_IMP_LIB_FILE}" LINKER_LANGUAGE C
+        PREFIX "" SUFFIX "" OUTPUT_NAME "${LIBOMP_IMP_LIB_FILE}" LINKER_LANGUAGE ${LIBOMP_LINKER_LANGUAGE}
         STATIC_LIBRARY_OPTIONS "${CMAKE_LINK_DEF_FILE_FLAG}${CMAKE_CURRENT_BINARY_DIR}/${LIBOMPIMP_GENERATED_DEF_FILE}")
     add_dependencies(${LIBOMP_IMP_LIB_TARGET} omp libompimp-needed-def-file)
   endif()
 endif()
 
 # Building the Fortran module files
 # One compilation step creates both omp_lib.mod and omp_lib_kinds.mod
 if(${LIBOMP_FORTRAN_MODULES})
   configure_file(${LIBOMP_INC_DIR}/omp_lib.h.var omp_lib.h @ONLY)
   configure_file(${LIBOMP_INC_DIR}/omp_lib.f90.var omp_lib.f90 @ONLY)