- only precompile boost and stl headers
- for qt4 also precompile QtGui and QtGui
- don't use one global precomiled header file
to get out of the dependency hell
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15517 a592a061-630c-0410-9148-cb99ea01b6c8