Update to docs etc.

[Paul Smith]

It's been pointed out on the GNU make lists that by including gnulib
content we've effectively updated our minimum required compiler from
C90 to C99.

That may be acceptable, but I think the gnulib docs (at least), if not
gnulib-tool, should make clear that AC_PROG_CC_C99 (at least) needs to
be added to to configure.ac.

In GNU make we only have AC_PROG_CC which was fine before we started
using gnulib but now causes failures on systems where this gives a C89
compiler, without extra options needed to choose C99.

Gnulib macros may also need to fail the configure if the compiler
cannot support C99.