We have find module wrappers for various system libs that use
FindPkgConfig. The _FOUND variable will be set to 1 if pkg-config
--exists returned true, but the imported target will not be created when
for example the provided library paths or header include paths are
outside of the sysroot - a good sanity check by cmake.
We require the targets for use though, so therefore we must unset the
_FOUND variable if we can't locate the targets.
Change-Id: Ia0dea657684a1847148664521265c42585890645
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
