fix: drop Qt4 qmake support

fix: drop Qt4 qmake support

It looks to me as if cdc5efaf40d94b1020b650d654f94ebf0a856c62 attempted to
consolidate Qt5 support but actually erroneously re-enabled Qt4 qmake support.
This re-enabled qmake doesn't work because it produces a Makefile designed to
work with Qt4, while the CFLAGS and LDFLAGS set by PKG_CHECK_MODULES are all
based on Qt5 library names.

The outcome of this is that building on a Linux machine with libqt5 and
libqt4-dev installed but *not* libqt5-dev (a legal configuration) causes
Graphviz compilation to fail when processing GVEdit #includes it cannot find.

The fix here is to simply remove support for Qt4's qmake because Qt4 is
deprecated [0] and force the build system to find Qt5's qmake. Closes #1862.

  [0]: https://www.qt.io/blog/2014/11/27/qt-4-8-x-support-to-be-extended-for-another-year