Replace 'or' with '||' and use QT's PKGCONFIG macro

Building on Windows, I could only get pkg-config to work by using qmake's [PKGCONFIG variable](http://qt-project.org/faq/answer/does_qmake_have_support_for_pkg-config).

The alternative and/or/etc. tokens in C++ are obscure and require certain flags or header files in at least one compiler.