Fix check for finding PkgConfig

This is a follow up of !1354 (merged). There PkgConfig is only looked up if the pkg_check_modules macro hasn't been defined. The problem is that the scope of macros seem to be global, and finding PkgConfig in another directory defines that macro. However, variable scope is per directory and the PkgConfig macros don't really work when PkgConfig is not found in the current directory.

This changes the check to use the PKG_CONFIG_FOUND variable