llvm-config: Include -stdlib= in --cxxflags

[tstellar]

Summary

This was removed in r349068, but it is needed when llvm is compiled
using the non-default c++ standard library on a platform.

llvm/tools/llvm-config/CMakeLists.txt

 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})
 
 # Use configure_file to create BuildVariables.inc.
 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} ${COMPILE_FLAGS} ${LLVM_DEFINITIONS}")
+set(LLVM_CXXFLAGS "${LLVM_CXX_STD_FLAG} ${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}")
 set(LLVM_HAS_GLOBAL_ISEL "ON")
 
 # Use the C++ link flags, since they should be a superset of C link flags.
 set(LLVM_LDFLAGS "${CMAKE_CXX_LINK_FLAGS}")
 set(LLVM_BUILDMODE ${CMAKE_BUILD_TYPE})