Link thread library to libpar_gc.

On Linux (and others) the threaded copy of Boehm GC that we build,
libpar_gc.so, inadvertently did not link to the POSIX thread library.
When producing an executable we link with both libpar_gc and libpthread
but recent versions of gcc/GNU ld have different default behaviour,
such that pthread symbol references in libpar_gc will NOT be resolved by
passing -lpthread at the executable linking step.  libpar_gc itself
needs to be linked to libpthread.  The error message looks like:

	/usr/bin/ld: test: hidden symbol `pthread_atfork' in
	/usr/lib/i386-linux-gnu/libpthread_nonshared.a(pthread_atfork.oS)
	is referenced by DSO

(Another workaround I found was to use the gcc -pthread option instead
of linking using -lpthread.  I don't know why that works.)

boehm_gc/Makefile.direct:
	Make the Mercury project-specific rule that builds libpar_gc.so
	link with $THREAD_LIBS that was set by configure.

	Add a comment about NOT using the threadlibs.c helper program.
Define GC_LINUX_THREADS instead of LINUX_THREADS.

Pass -DGC_LINUX_THREADS for Boehm GC instead of -DLINUX_THREADS.
The latter is a deprecated synonym.

configure.ac:
	As above.

*/RESERVED_MACRO_NAMES:
	LINUX_THREADS no longer needs to be excluded from the
	namespace check.