[CMake] Accessing -fPIC and -std settings within CMake definitions

[Stephen Morris]

I'm setting up a custom build step to build a precompiled header in gcc (yes I know that native support is coming with CMake 3.16, but I need to get something working on an older version).

My approach is basically to set up a custom command thus:

set_target_properties(mytarget PRIVATE CXX_STANDARD 17)
set_target_properties(mytarget PRIVATE  POSITION_INDEPENDENT CODE_1)
set(CMAKE_VERBOSE_MAKEFILE TRUE)
...
...
set(CXX_FLAGS ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_DEBUG})   .. or whatever, depending on the current configuration..
get_target_property(compile_options, mytarget, COMPILE_OPTIONS)
add_custom_command(OUTPUT myheader.h.gch
                                           COMMAND ${CMAKE_CXX_COMPILER} ${CXX_FLAGS} ${compile_options} -fPIC -std=gnu++17 -c myheader.h -o myheader.h.gch
                                           DEPENDS myheader.h)
add_custom_target(BuildMyPCH
                                     DEPENDS myheader.h.gch)
add_dependencies(mytarget, BuildMyPCH)

You'll note that in my custom command I've had to specify -fPIC and -std-gnu++17 explicitly, because they aren't included among either CXX_FLAGS or COMPILE_OPTIONS. I've looked for them among the  COMPILE_DEFINITIONS, COMPILE_FEATURES and COMPILE_FLAGS properties of my target as well, but there's no sign of them. And yet if I look at the verbose output when gcc builds other files using the CMake-generated makefile, I see that they are being included automatically.

So, my question: where is CMake storing these settings, and how can I apply them in my custom command without having to do so manually?