The linux hints file should only look for -lgdbm_compat if -lgdbm is wanted.

Previously they would unconditionally add gdbm_compat to the list of wanted
libraries. This is unhelpful if the user has passed Configure arguments to
constrain the library search to avoid libgdbm.so, because Configure would
still end up finding libgdbm_compat.so, which at best is not useful without
libgdbm.so, and at worse could have the same problems that caused the
user to avoid libgdbm.so (eg not compatible with the current compiler flags)