Update patch for posix threads for poppler

* patches/poppler/
0001-Explicitly-set-pthread-for-gcc-windows-builds.patch: Removed.
* patches/poppler/
0001-Explicitly-take-posix-variant-for-mingw-gcc-cross.patch: New.
* Makefile.am (EXTRA_DIST): Update accordingly.

This appears to be more hacky then required according through
cmake documentation "FindThreads" should be used. But this just
did not work. Maybe because of the update-alternative usage?
So we now force the compiler name.