I have spent a while investigating this and this is indeed pretty tricky to carry CXX_STANDARD to PCH target. The problem that current CMake implementation for selecting C standard is pretty complicated and uses combination of CMAKE_CXX??_STANDARD/EXTENSION_COMPILE_OPTION compiler defined variables and custom dedicated code of cmLocalGenerator::AddCompilerRequirementFlag that adds these flags.

I tried to convince CMake to add these for PCH target too adding relevant entries in CMakeCXXPCHCompiler.cmake.in as it check if the compiler has CMAKE_ lang _STANDARD_DEFAULT defined, but it seems to be not enough.

FYI current baked in solution provided by @mick-p1982 simply bakes -std=gnu <STD> rather using the CMakes built-in complicated C standard flags handling. That is why you get wrong standard. For now you can simply replace -std=gnu with -std=c and I believe this should do the job for you.

I am rather short on time for this project and already spend couple of hours yesterday, so any help would be appreciated.

Hm. I think maybe it is possible to use CMAKE_EXPORT_COMPILE_COMMANDS to get exact compile command? It will work only with Ninja and Unix Makefile generators. I tried it with my project so output:

[
{
  "directory": "/home/taurus/develop/torrent-file-editor/release-qt5",
  "command": "/usr/lib64/ccache/g    -DHAVE_QT5 -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NO_CAST_FROM_ASCII -DQT_NO_DEBUG -DQT_WIDGETS_LIB -I../ -I. -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtCore -isystem /usr/lib64/qt5/mkspecs/linux-g   -isystem /usr/include/qt5/QtGui -isystem /usr/include/qt5/QtWidgets   -std=c  11 -Wall -Wextra -pedantic -Werror -Wno-error=cpp -O2 -DNDEBUG   -fPIC -std=gnu  11 -o CMakeFiles/torrent-file-editor.dir/application.cpp.o -c /home/taurus/develop/torrent-file-editor/application.cpp",
  "file": "/home/taurus/develop/torrent-file-editor/application.cpp"
}
]

If there is no way to detect it with CMake then there has to be a parameter added to module's commands.

I should add that C XX should be a subset of gnu XX. The module is supposed to just give your what you need to do PCH with CMake for popular platforms. If switching to gnu standard is a bridge too far, then you don't really require PCH for your project!