Replace m4/warning.m4 with warnings module.

* bootstrap.conf: Replace m4/warning.m4 with warnings module.
* configure.ac: Adjust usage.
* lib/Makefile.am: Replace $(WARNING_CFLAGS) with $(WARN_CFLAGS).
* src/Makefile.am: Replace $(WARNING_CFLAGS) with $(WARN_CFLAGS).
* tests/atlocal.in: Replace $(WARNING_*FLAGS) with $(WARN_*FLAGS).

m4/.gitignore

@@ -58,6 +58,7 @@
 /unistd_h.m4
 /unlocked-io.m4
 /warning.m4
+/warnings.m4
 /wchar.m4
 /wchar_t.m4
 /wctype.m4