extra compiler CONFIG+=target_predeps does not work with vcproj output

Currently we sadly miss a qmake variable that allows specifying a custom pre-build step. There are QMAKE_POST_LINK and QMAKE_PRE_LINK but those do not help if something needs to be generated before compilation.

We need to generate version information headers for our projects using a script file. This of course must be done before compilation. Since we use generic .pro files to produce *.vcxproj projects for VS we can not modify the VS projects manually.

Please implement a QMAKE_PRE_BUILD variable to close this gap.