- host GSL libs are sometimes found leading to errors
reported by buildbot, replicated locally (Arch Linux)

Error:
configure: GSL_LIBS=-L/usr/lib -lgsl -lgslcblas -lm
leading to
/usr/include/features.h:398:23: fatal error: gnu/stubs.h: No such file or directory

- explicit libdb location to prevent host includes

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>