Remove override template hacks

* src/Makefile.am: Use qtpkg template. Update qt package defines.
* src/gpg4win.mk.in: Add boost template and qt package template.

The override of targets was a hack and led to warnings and it
also did not simplify things much. Now we have a bit more template
duplication but things are simpler.
Especially boost build was extremly fragile with the configure
target creating the make stamps.