[CMake] CMake, precompiled headers, Apple, gcc and Qt

[Dieter Oberkofler]

Hi!

I'm (once again) struggling with the creation of precompiled headers in
conjunction with gcc and Qt on the Apple platform.

When creating my precompiled header I use a code section based on
"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:

1) Is the above way to retrieve the compiler flags correct ?
2) Is there a better, easier, simpler way to do this ?
3) Why does -DQT_DEBUG not show up in COMPILE_DEFINITIONS or
COMPILE_DEFINITIONS_${CMAKE_BUILD_TYPE}

Thank you for your help in advance.
D