packagesExist(): always return false if QT_CONFIG contains no-pkg-config

packagesExist(): always return false if QT_CONFIG contains no-pkg-config

On QNX e.g. you'll end up with a Qt configured without pkg-config support
by default. Once you try to compile e.g. QtWebKit which contains a check
like packagesExist(libudev) the system pkg-config (see default branch
of pkgConfigExecutable) will be called without any special config sysroot
or libdir. Thus libudev will be found in your system host, even though
it is not available for the cross compile target. This leads to compile
errors as described in https://bugs.webkit.org/show_bug.cgi?id=98032 .

Change-Id: If1474cb3b3f3b71fad7269e4aedd4e10f70a9b2a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>