RFC: Changing AC_PROG_CC to AC_PROG_CC_C99 in top level configure

[Simon Marchi]

> Yes, I prefer the configure fix too.  If we state we require C99 in
> binutils then we ought to be able to use C99..
> 
> Nick, does the configure.ac change also need to go in all subdirs, to
> support people running make in say ld/ rather than running make in the
> top build dir?

For GDB, it's not supported to run gdb/configure directly, you need to
use the top-level configure.  Is it supported from some of the other
projects in the repo?

I just tried with ld, it doesn't work since it depends on bfd also being
built.  I tried with just bfd, it doesn't work (with the default
configure options at least) because it requires zlib being built.

So if all projects need to go through the top-level configure script
anyway, and C99 is a baseline for all projects, then having the check
only in the top-level makes sense to me.  Projects that have more
specific requirements can have their own checks.  For example, sim/
requires C11 now.  Unless the C99 check at top-level somehow does not
play well with the C11 check in sim/?  Like if that would cause CC to be
set to "gcc -std=gnu99 -std=gnu11" or something like that.

Simon