llvm-config: emit the C++ standard flag into CXXFLAGS

This recovers the now "missing" flag as this is controlled by CMake
rather than injected into the user defined flags list.  This is
primarily needed by LDC and other out-of-tree users which do not
correctly setup the C++ flags.

Author: compnerd

llvm/tools/llvm-config/CMakeLists.txt

@@ -29,12 +29,9 @@ string(REPLACE ";" " " SYSTEM_LIBS "${SYSTEM_LIBS}")
 # Fetch target specific compile options, e.g. RTTI option
 get_property(COMPILE_FLAGS TARGET llvm-config PROPERTY COMPILE_FLAGS)
 
-# The language standard potentially affects the ABI/API of LLVM, so we want
-# to make sure it is reported by llvm-config.
 # NOTE: We don't want to start extracting any random C/CXX flags that the
 # user may add that could affect the ABI.  We only want to extract flags
 # that have been added by the LLVM build system.
-string(REGEX MATCH "-std=[^ ]+" LLVM_CXX_STD_FLAG ${CMAKE_CXX_FLAGS})
 string(REGEX MATCH "-stdlib=[^ ]+" LLVM_CXX_STDLIB_FLAG ${CMAKE_CXX_FLAGS})
 string(REGEX MATCH "-std=[^ ]+" LLVM_C_STD_FLAG ${CMAKE_C_FLAGS})
 
@@ -43,7 +40,9 @@ set(LLVM_SRC_ROOT ${LLVM_MAIN_SRC_DIR})
 set(LLVM_OBJ_ROOT ${LLVM_BINARY_DIR})
 set(LLVM_CPPFLAGS "${LLVM_DEFINITIONS}")
 set(LLVM_CFLAGS "${LLVM_C_STD_FLAG} ${LLVM_DEFINITIONS}")
-set(LLVM_CXXFLAGS "${LLVM_CXX_STD_FLAG} ${LLVM_CXX_STDLIB_FLAG} ${COMPILE_FLAGS} ${LLVM_DEFINITIONS}")
+# The language standard potentially affects the ABI/API of LLVM, so we want
+# to make sure it is reported by llvm-config.
+set(LLVM_CXXFLAGS "${CMAKE_CXX${CMAKE_CXX_STANDARD}_STANDARD_COMPILE_OPTION} ${LLVM_CXX_STDLIB_FLAG} ${COMPILE_FLAGS} ${LLVM_DEFINITIONS}")
 set(LLVM_BUILD_SYSTEM cmake)
 set(LLVM_HAS_RTTI ${LLVM_CONFIG_HAS_RTTI})
 set(LLVM_DYLIB_VERSION "${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX}")