supposing that they should set SHLIB_LINK rather than LDFLAGS_SL.  Since these
don't go through Makefile.shlib that was a no-op on most platforms.  Also
regularize the few platform-specific Makefiles that did pay attention to
SHLIB_LINK: it seems that the real value of that is to pull in BE_DLLLIBS,
so do that instead.  Per buildfarm failures on cygwin.