Add support for --enable-devel and similar macros.

* m4/debug.m4: Rename to ...
* m4/bdebug.m4: ... this.
* m4/debug.m4, m4/devel.m4, m4/gccoptim.m4, m4/ndebug.m4: New file.
* m4/gccwarns.m4: Fix usage of cache variable.  Fix shell
syntax.  Do not check for -Waggregate-return.  Update CFLAGS.
* configure.ac: Adjust to handle --enable-devel and similar macros
in the same way as Spot.