Remove build_atomic_ops.sh

* Makefile.am (EXTRA_DIST): Remove build_atomic_ops.sh,
build_atomic_ops.sh.cygwin entries.
* Makefile.direct (OTHER_MAKEFILES): Likewise.
* Makefile.direct (AO_SRC_DIR): Update comment.
* Makefile.direct (AO_INSTALL_DIR): Remove variable.
* Makefile.direct (CFLAGS_EXTRA): New variable (empty by default).
* Makefile.direct (CFLAGS, SPECIALCFLAGS): Replace
"$(AO_INSTALL_DIR)/include" with "$(AO_SRC_DIR)/src" include folder.
* Makefile.direct (CFLAGS): Add $(CFLAGS_EXTRA).
* Makefile.direct (OBJS): Add atomic_ops.o, atomic_ops_sysdeps.o
entries.
* Makefile.direct (AO_INSTALL_DIR): Remove rule (build essential .o
files of libatomic_ops directly instead of configure and make).
* Makefile.direct (atomic_ops.o, atomic_ops_sysdeps.o): Add rule.
* Makefile.direct (tests/test.o, setjmp_test): Remove dependency on
$(AO_INSTALL_DIR).
* Makefile.direct (gc.a): Do not copy contents of libatomic_ops.a to
gc.a.
* Makefile.direct (sunos5gc.so): Do not link with libatomic_ops.a.
* Makefile.direct (gc.tar): Do not make libatomic_ops.
* build_atomic_ops.sh: Remove file.
* build_atomic_ops.sh.cygwin: Likewise.
Fix unresolved vsnprintf in misc.c and snprintf in cordtest (DJGPP, VC)

* cord/tests/cordtest.c (GC_SNPRINTF, GC_SNPRINTF_BUFSZ_ARG): New
macro to workaround snprintf() missing in DJGPP and MS VC.
* cord/tests/cordtest.c (test_printf): Replace snprintf() with
GC_SNPRINTF and GC_SNPRINTF_BUFSZ_ARG.
* misc.c (GC_VSNPRINTF): Test DJGPP instead of NO_VSNPRINTF; refine
comment.
Remove Makefile.dj (and README.dj) in favor of configure (DJGPP)

* Makefile.am (EXTRA_DIST): Remove Makefile.dj entry.
* Makefile.direct (OTHER_MAKEFILES): Likewise.
* Makefile.direct (DOC_FILES): Remove doc/README.dj entry.
* doc/doc.am (dist_pkgdata_DATA): Likewise.
* Makefile.dj: Remove file ("configure" is to be used to build libgc,
no special options needed).
* doc/README.dj: Likewise.