cmake/sip_configure.py: Use of non-standard qmake binary names not completely solved

[a17r]

This was already touched in #22 but not really solved.

Whatever you think will be in PATH for qmake, it is not standardised at all prior to Qt6. In Qt6, upstream finally decided to make Qt installable side-by-side using 6 suffix on respective Qt binaries, and they also introduced a distinction on what would be "user-facing" (in PATH) or not (remain in some lib/libexec subdir not in PATH). In Gentoo we decided to do the same with Qt5, we had already done a great deal of work in past years to fix packages not to depend on a random filename pattern search in PATH.

Therefore:
qmake may be any of binary, hardlink, softlink controlled by qtchooser, which may be switched at will to any major Qt version by the user through config file
Qt4 may be qmake, qmake4, qmake-qt4, qt4-qmake, and those are just variants I know of
Qt5 same as above, random across distros.

The cleanest way to solve this is give configure the possibility to have the QTBINDIR injected, which can then be set by distro accordingly.