Fix monolithic build in cmake (wxBUILD_MONOLITHIC)

- Functions that set wxMONO_* vars need to set them in parent scope, from macros instead of functions (functions would need grandparent scope)
- Fix a conflict with xml library linking Mono which made core include the wrong png headers
- Fix setting of NOPCH on windows to go through the wx_lib_compile_definitions instead of target_compile_definitions so it carries to monolithic build
- Allow use of precompiled header in MSVC via calling wx_finalize_lib(mono)
- Introduce wxMONO_NONCOMPILED_CPP_FILES to deal with set_source_files_properties only applying within the current CMakeLists file
- Fix demos building against monolithic library by using wx_exe_link_libraries instead of target_link_libraries (similar to what the tests already do)

Closes #18074

build/cmake/demos/CMakeLists.txt

@@ -35,7 +35,7 @@ function(wx_add_demo name)
     if(wxBUILD_SHARED)
         target_compile_definitions(${DEMO_NAME} PRIVATE WXUSINGDLL)
     endif()
-    target_link_libraries(${DEMO_NAME} core ${DEMO_LIBRARIES})
+    wx_exe_link_libraries(${DEMO_NAME} core ${DEMO_LIBRARIES})
     wx_set_common_target_properties(${DEMO_NAME})
     set_target_properties(${DEMO_NAME} PROPERTIES FOLDER "Demos")
     set_target_properties(${DEMO_NAME} PROPERTIES