libtool: only strip -Wl when linking with $LD.

* build-aux/ltmain.in (func_mode_link): Don't strip -Wl from the
linker flags, except when linking with $LD, otherwise compiler
drivers that don't recognize and pass linker flags through will
choke on unrecognized options.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>