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.