Correctly check for and use Qt4's qmake

Instead of just using qmake, check if either qmake or qmake or qmake-qt4
exists and uses version 4 of Qt. This affects both the dependency check
and the actual compilation of Dwarf Therapist.

This also generally fixes the dependency check. Previously, it actually
tested for Mercurial a second time.
Query QT_VERSION instead of using qmake -v
Separate qmake detection from dependency check

find_qmake_qt4 is now a separate function called by
build_dwarf_therapist and check_dependencies.
Revbump to 0.2.4, fixing Qt4 detection.