qmake: Improve Qt5 build support.

Qt5 modules built from git rely on git information being available to
generate private headers and the like, so don't strip out the .git
directly when using builddir hack.

Likewise, using the "-r" flag to force a full recursive search for .pro
files causes qmake to error out on some modules where the .pro isn't
meant to be built as part of the module build (e.g. examples).