[libcxx] Allow target flags to affect CMake configuration tests

[EricWF]

Summary

This patch changes the libc++ CMake so that it adds certain target flags like '-m32' or '--gcc-toolchain' before including config-ix.cmake.
Since these flags can affect things like check_library_exists([...]) they needed to be added before the tests are performed.

This patch fixes:

https://llvm.org/bugs/show_bug.cgi?id=24322

[compnerd]

CMakeLists.txt

259

This is missing the `='. As much as I like the separate arguments, it does cause a difference here :-(. It should be "--sysroot=${LIBCXX_SYSROOT}".

[EricWF]

Address review comments. Add "=" sign in sysroot.

CMakeLists.txt

 # They are intended for use as follows:
 # LIBCXX_CXX_FLAGS: General flags for both the compiler and linker.
 # LIBCXX_COMPILE_FLAGS: Compile only flags.
 # LIBCXX_LINK_FLAGS: Linker only flags.
 set(LIBCXX_COMPILE_FLAGS "")
 set(LIBCXX_LINK_FLAGS "")
 set(LIBCXX_LIBRARIES "")
 
+# Include macros for adding and removing libc++ flags.
+include(HandleLibcxxFlags)
+
+# Target flags ================================================================
+# These flags get added to CMAKE_CXX_FLAGS and CMAKE_C_FLAGS so that
+# 'config-ix' use them during feature checks. It also adds them to both
+# 'LIBCXX_COMPILE_FLAGS' and 'LIBCXX_LINK_FLAGS'
+add_target_flags_if(LIBCXX_BUILD_32_BITS "-m32")
+add_target_flags_if(LIBCXX_TARGET_TRIPLE "-target ${LIBCXX_TARGET_TRIPLE}")
+add_target_flags_if(LIBCXX_SYSROOT "--sysroot=${LIBCXX_SYSROOT}")
+add_target_flags_if(LIBCXX_GCC_TOOLCHAIN "-gcc-toolchain ${LIBCXX_GCC_TOOLCHAIN}")
+
 # Configure compiler.
 include(config-ix)
 
 # Configure coverage options.
 if (LIBCXX_GENERATE_COVERAGE)
   include(CodeCoverage)
   set(CMAKE_BUILD_TYPE "COVERAGE" CACHE STRING "" FORCE)
 endif()
 
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 
 #===============================================================================
 # Setup Compiler Flags
 #===============================================================================
 
 include(HandleLibCXXABI) # Setup the ABI library flags
 
-# Include macros for adding and removing libc++ flags.
-include(HandleLibcxxFlags)
-
 # Remove flags that may have snuck in.
 remove_flags(-DNDEBUG -UNDEBUG -D_DEBUG
              -stdlib=libc++ -stdlib=libstdc++ -lc++abi -m32)
 
 # FIXME(EricWF): See the FIXME on LIBCXX_ENABLE_PEDANTIC.
 # Remove the -pedantic flag and -Wno-pedantic and -pedantic-errors
 # so they don't get transformed into -Wno and -errors respectivly.
 remove_flags(-Wno-pedantic -pedantic-errors -pedantic)
 
 # Required flags ==============================================================
 add_compile_flags_if_supported(-std=c++11)
 if (NOT MSVC AND NOT LIBCXX_SUPPORTS_STD_EQ_CXX11_FLAG)
   message(FATAL_ERROR "C++11 is required but the compiler does not support -std=c++11")
 endif()
 
 # On all systems the system c++ standard library headers need to be excluded.
 # MSVC only has -X, which disables all default includes; including the crt.
 # Thus, we do nothing and hope we don't accidentally include any of the C++
 # headers
 add_compile_flags_if_supported(-nostdinc++)
 
-# Target flags ================================================================
-add_flags_if(LIBCXX_BUILD_32_BITS -m32)
-add_flags_if(LIBCXX_TARGET_TRIPLE "-target ${LIBCXX_TARGET_TRIPLE}")
-add_flags_if(LIBCXX_SYSROOT "--sysroot ${LIBCXX_SYSROOT}")
-add_flags_if(LIBCXX_GCC_TOOLCHAIN "-gcc-toolchain ${LIBCXX_GCC_TOOLCHAIN}")
 
 # Warning flags ===============================================================
 add_definitions(-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 add_compile_flags_if_supported(
     -Wall -W -Wwrite-strings
     -Wno-unused-parameter -Wno-long-long
     -Werror=return-type)
 if (LIBCXX_ENABLE_WERROR)

cmake/Modules/HandleLibcxxFlags.cmake

macro(config_define_if_not condition def)
   endif()
 endmacro()
 
 macro(config_define value def)
   set(${def} ${value})
   set(LIBCXX_NEEDS_SITE_CONFIG ON)
 endmacro()
 
+# Add a list of flags to all of 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS',
+# 'LIBCXX_COMPILE_FLAGS' and 'LIBCXX_LINK_FLAGS'.
+macro(add_target_flags)
+  foreach(value ${ARGN})
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${value}")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${value}")
+    list(APPEND LIBCXX_COMPILE_FLAGS ${value})
+    list(APPEND LIBCXX_LINK_FLAGS ${value})
+  endforeach()
+endmacro()
+
+# If the specified 'condition' is true then add a list of flags to
+# all of 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS', 'LIBCXX_COMPILE_FLAGS'
+# and 'LIBCXX_LINK_FLAGS'.
+macro(add_target_flags_if condition)
+  if (${condition})
+    add_target_flags(${ARGN})
+  endif()
+endmacro()
+
 # Add a specified list of flags to both 'LIBCXX_COMPILE_FLAGS' and
 # 'LIBCXX_LINK_FLAGS'.
 macro(add_flags)
   foreach(value ${ARGN})
     list(APPEND LIBCXX_COMPILE_FLAGS ${value})
     list(APPEND LIBCXX_LINK_FLAGS ${value})
   endforeach()
 endmacro()