src/include/builddefs.in: rework qmake recipe
src/include/builddefs.in: rework qmake recipe

Fixes:

- clobbering Makefile when nothing has changed
- sudo make install leaving behind a Makefile owned by root

Both are long-standing itches.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp