Qt5 build support

Now ready for review.

Noteworthy changes:
- --with-qt now accepts a version. --with-qt=no | qt4 | qt5 | auto.
- If a version is not specified, it will use the first one it can find. Qt4 is searched before Qt5, this will probably be reversed at some point.
- Ubuntu/Mingw/OSX builds are confirmed working fine.
- Fixed the search order of --with-qt-bindir (#3063). If that dir is specified, it searches for any usable tool before searching elsewhere.

Using:
Linux: ./configure --with-qt=qt5
Windows: Specify --with-qt-incdir, --with-qt-libdir, --with-qt-plugindir, --with-qt-bindir
OSX: ./configure PKG_CONFIG_PATH=/Users/cory/Qt5.0.2/5.0.2/clang_64/lib/pkgconfig --with-qt=qt5 --with-qt-bindir=/Users/cory/Qt5.0.2/5.0.2/clang_64/bin

OSX is tricky. Because there's no macports build yet, I've tested with a binary download I had laying around. For now, this is how we'll use it.