Fix bug #203: install to temporary location with DESTDIR.

Attempting to install Mercury into a temporary location, with DESTDIR
set, would:

- wrongly install libatomic_ops into DESTDIR

- be unable to install library grades because the installed mmc script
  expects files in their final locations, without the DESTDIR prefix.

Thanks to Jeremy Huddleston and Nikolay Orlyuk for reporting the bug,
and an earlier version of this patch.

Mmakefile:
	Override MERCURY_COMPILER and MERCURY_CONFIG_DIR when installing
	library grades.

boehm_gc/build_atomic_ops.sh:
	Clear DESTDIR so that libatomic_ops is installed into the
	boehm_gc directory, as intended.