Make it possible to build bdwgc as part of Mercury

When building bdwgc (from git) as part of Mercury, autotools does not
properly separate bdwgc's configuration from Mercury's.  The result is that
some files are created in the parent project but bdwgc's build system looks
for them in the boehm_gc directory.

This change works around the problem by explicitly configuring libtool in
the boehm_gc directory.

configure.ac:
    Configure libtool to use the current directory.

autogen.sh:
    Call libtoolize.

build_atomic_ops.sh:
build_atomic_ops.sh.cygwin:
    Call autogen.sh in the libatomic_ops directory.