d: respect --enable-link-mutex configure option

I noticed the option is ignored because @DO_LINK_MUTEX@
is not defined in d/Make-lang.in.

gcc/ChangeLog:

	* Makefile.in: Set DO_LINK_MUTEX.

gcc/d/ChangeLog:

	* Make-lang.in: Use it as $DO_LINK_MUTEX.
2 files changed