hs-boehmgc/buildsystem: rm of tempdir after cabal install caused problems with foreig...

hs-boehmgc/buildsystem: rm of tempdir after cabal install caused problems with foreign libs (libgc not found during linking); this occurs due to linking with mate - altough  cabal Setup.hs patches correct extra-library-dirs these dirs dont exist after build;

there are two possible fixes:
- ship hs-boehmgc with libgc
- patch cabal install dirs into cabal file in Setup.hs