Using CONFIG += link_pkgconfig gives weird errors when Qt is configured without pkg-config suppoort

I don't know the reasons why Qt tries to detect at build time if pkgconfig is present. This is usually false on Mac builds, hence no-pkg-config gets added to QT_CONFIG.

If pkgconfig is then installed, the user expects CONFIG += link_pkgconfig and PKGCONFIG = foo to "just work". Instead, Qt says "Package foo not found" (from link_pkgconfig.prf).

At least one should do an explicit test for contains(QT_CONFIG, no-pkg-config) there and not let packagesExist fail... but why do we require that "build time pkgconfig support" in order to use pkgconfig? Let's instead fail if the binary is not in PATH...