Currently, the FindQt5PlatformSupport and FindQt5ThemeSupport cmake modules use pkg-config to search for the includes files without searching if those files would be available in the default path first (for instance if building against a custom Qt installation and using CMAKE_PREFIX_PATH, and not the version installed on the system).
This change searches for the header files in the default path first, then defaults to the current behavior of using pkg-config.
This becomes a blocking issue with Qt5.11.1 as the prototype of the constructor of some classes changed in QtThemeSupport vs Qt 5.9. The library found would be 5.11.1 (CMAKE_PREFIX_PATH) but the header files would be 5.9 (installed on the system and found by pkg-config).