qmake: pkg-config checks with link_pkgconfig don't fail

When using link_pkgconfig to depend on a library, like so:

CONFIG += link_pkgconfig
PKGCONFIG += somelibrary

qmake does not fail if the library is not found (if the .pc file does not exist). So we just see a compilation error when the header can't be found. It should fail as soon as the depencency can't be found.

In the unusual case, some projects might want to ifdef their code depending on whether the dependency existed, but that's not the common case, and I don't see any way to detect it in code when using qmake anyway.