message: Fix pch generation when linker flags used with GCC
  c is required to avoid linking (and the associated missing "main" message)
  when linker flags like "-Wl,-z,relro" are passed to GCC
diff: @@ -125,11 +125,11 @@ MACRO(_PCH_GET_COMPILE_COMMAND out_command _input _output)
  STRING(REGEX REPLACE "^ +" "" pchsupport_compiler_cxx_arg1 ${CMAKE_CXX_COMPILER_ARG1})
  SET(${out_command}
  - ${CMAKE_CXX_COMPILER} ${pchsupport_compiler_cxx_arg1} ${_compile_FLAGS} -x c++-header -o ${_output} ${_input}
  + ${CMAKE_CXX_COMPILER} ${pchsupport_compiler_cxx_arg1} ${_compile_FLAGS} -x c++-header -o ${_output} -c ${_input}
  )
  ELSE(CMAKE_CXX_COMPILER_ARG1)
  SET(${out_command}
  - ${CMAKE_CXX_COMPILER} ${_compile_FLAGS} -x c++-header -o ${_output} ${_input}
  + ${CMAKE_CXX_COMPILER} ${_compile_FLAGS} -x c++-header -o ${_output} -c ${_input}
  )
  ENDIF(CMAKE_CXX_COMPILER_ARG1)
  ELSE()
  