Override all per-target *_LINK variables correctly.
boehm-gc/:
* Makefile.am (libgcjgc_la_LINK, gctest_LINK): New.
(gctest_LDADD): Depend on libgcjgc.la instead of ./libgcjgc.la,
so that library dependency resolution works with portable make.
* Makefile.in: Regenerate.