Compile checksums.c only if --with-checksums is given (configure)

Compile checksums.c only if --with-checksums is given (configure)

New configure option (--with-checksums) is added, mainly to workaround
'checksums.a has no symbols' link warning (observed on Darwin).

* Makefile.am [!SINGLE_GC_OBJ] (libgc_la_SOURCES): Do not add checksums.c
unless CHECKSUMS.
* configure.ac (checksums): New AC_ARG_WITH.
* configure.ac [with_checksums && (enable_munmap || THREADS!=none)]:
Issue AC_MSG_ERROR.
* configure.ac (CHECKSUMS): New AC_DEFINE and AM_CONDITIONAL if
with_checksums.