The plan is to update the c99 module to depend on std-gnu23
instead of on std-gnu11, and to make std-gnu11 obsolete.
A benefit of this will be that c99 will no longer affect C++.
For now, though, std-gnu23 is simply a new, optional module.
* MODULES.html.sh (func_all_modules): Add it.
* modules/std-gnu11: Change description.
* modules/std-gnu23, m4/std-gnu23.m4: New files.
* m4/std-gnu11.m4: Do nothing if _AC_C_C23_OPTIONS is defined.
(AC_PROG_CC): Use AC_DEFUN, not AC_DEFUN_ONCE, so that
we can be superseded.
This makes it easier for applications to prefer C11 and C++11
to older variants, when compiling C and C++ code.
Unlike most m4/*.m4 files, m4/std-gnu11.m4 is GPLed, as it copies
a nontrivial chunk of GPLed Autoconf source code.
* COPYING: Mention the m4/*.m4 copyright situation.
* MODULES.html.sh (std-gnu11): New module.
* m4/std-gnu11.m4, modules/std-gnu11: New files.
