Summary: If we create a new temp directory for each build, scons will recompile everything because we have different parameters. Instead, let's set up a constant path to our static lib. That way we won't have to recompile.
Test Plan: Run fb_compile_mongo.sh twice -- second time it didn't recompile everything
Reviewers: MarkCallaghan, anthony
Reviewed By: anthony
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D40707
472e64d39e