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.
