BUILDING.txt -> BUILDING.rst

Also add qt4 instructions.
This actually doesn't work for me in qt-4.8.5.  It doesn't accept -qt as a valid parameter.  I just run qmake without arguments.

In fact qt-5.3.1's qmake doesn't seem to accept -qt either.  I just run qmake without arguments there as well.  The makefile is correctly configured for qt4 if I run qt4's qmake and is correctly configured for qt5 is I run qt5's qmake.