Fix linkage with a system libatomic_ops shared library

Fix linkage with a system libatomic_ops shared library

Issue #247 (bdwgc).

When bdwgc is linked with the external libatomic_ops, bdw-gc.pc must
contain the needed dynamic libraries (such as -latomic_ops) otherwise
build of applications could fail on the link stage on some hosts:
* libgc.so: undefined reference to 'AO_fetch_compare_and_swap_emulation'
* libgc.so: undefined reference to 'AO_store_full_emulation'

So, this commit sets ATOMIC_OPS_LIBS to "-latomic_ops" when a system
atomic_ops library is used and uses ATOMIC_OPS_LIBS in bdw-gc.pc.in.

* bdw-gc.pc.in (Libs): Add @ATOMIC_OPS_LIBS@.
* configure.ac [$with_libatomic_ops!=no && $with_libatomic_ops!=none
&& $THREADS!=none] (ATOMIC_OPS_LIBS): Set to -latomic_ops; do AC_SUBST.