CMake, Qt, gcc and precompiled headers - qt

Question

I'm (once again) struggling with the creation of precompiled headers in conjunction with gcc and Qt on the Apple platform.
When now creating my precompiled header I use a code section (based on good old "PCHSupport_26.cmake") to extract the compile flags as follows:
STRING(TOUPPER "CMAKE_CXX_FLAGS_${CMAKE_BUILD_TYPE}" _flags_var_name)
SET(_args ${CMAKE_CXX_FLAGS} ${${_flags_var_name}})
GET_DIRECTORY_PROPERTY(DIRINC INCLUDE_DIRECTORIES )
FOREACH(_item ${DIRINC})
LIST(APPEND _args "-I${_item}")
ENDFOREACH(_item)
GET_DIRECTORY_PROPERTY(_defines_global COMPILE_DEFINITIONS)
LIST(APPEND defines ${_defines_global})
STRING(TOUPPER "COMPILE_DEFINITIONS_${CMAKE_BUILD_TYPE}" _defines_for_build_name) GET_DIRECTORY_PROPERTY(defines_build ${_defines_for_build_name})
LIST(APPEND _defines ${_defines_build})
FOREACH(_item ${_defines})
LIST(APPEND _args "-D${_item}")
ENDFOREACH(_item ${_defines})
LIST(APPEND _args -c ${CMAKE_CURRENT_SOURCE_DIR}/${PRECOMPILED_HEADER} -o ${_gch_filename})
SEPARATE_ARGUMENTS(_args)
Unfortunately the above compiler flags miss two important parameter that CMake does generate when using the build-in compiler rules:
-DQT_DEBUG
and when compiling with the generated precompiled header, I get errors as follows:
file.h: not used because QT_DEBUG is defined.
I would need your help with the following:
Is the above way to retrieve the compiler flags correct ?
Is there a better, easier, simpler way to do this ?
Why does -DQT_DEBUG not show up in COMPILE_DEFINITIONS or COMPILE_DEFINITIONS_${CMAKE_BUILD_TYPE}

Answer 1

If you are using XCode simply:
SET_TARGET_PROPERTIES(${target} PROPERTIES XCODE_ATTRIBUTE_GCC_PRECOMPILE_PREFIX_HEADER YES)
SET_TARGET_PROPERTIES(${target} PROPERTIES XCODE_ATTRIBUTE_GCC_PREFIX_HEADER "${target}/std.h")
I'm trying myself to use precompiled headers with raw gcc on linux through CMake but I haven't still figured it out. It look like it's working but I don't see any speed improvements.
Edit:
I managed to use pch in gcc finally with the macro you can find here: http://www.mail-archive.com/cmake#cmake.org/msg04394.html