build: Add ability to bypass pkg-config for Qt

[hebasto]

Besides Windows platform, pkg-config works well for both system Qt and Qt in depends.

But there are some pitfalls when using different Qt versions that are downloaded with Qt Online Installer, e.g.:

• Build with (official binary installer) Qt 5.6 on OS X not supported #7714
• QTBUG-73151
• QTBUG-80922

Currently, there is no way to bypass pkg-config workflow for downloaded Qt (on non-Windows platforms). I have to use a patch to bypass pkg-config on testing purpose.

It seems to be useful for developers and testers to have an additional configure option, say --with-qt-dir, which forces to bypass pkg-config workflow.

For example:

./configure --with-gui=qt5 --with-qt-dir=/home/hebasto/Qt/5.14.0/gcc_64

More refs:

• Fix pkg-config issues for 0.13 #8314
• build: various build system improvements #17768