Qt5 build - Work in progress

This is nowhere near ready for use yet, but PRing for the sake of central discussion. Several devs have complained that autotools broke their ability to build with qt5, this adds it back.

I believe it's far enough along for linux/win32 devs to use while I begin cleaning it up.
Addresses: #3120 and #3063

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.