autoconf test for C `restrict' keyword

[Jim Meyering]

Is there any reason not to add a macro, AC_C_RESTRICT,
to handle the `restrict' keyword, along the same lines
as it already handles inline, const, volatile, etc.?

I found a macro, ACX_C_RESTRICT, by Steven G. Johnson
and have just tweaked that and added it to the GNU coreutils.

  http://article.gmane.org/gmane.comp.gnu.core-utils.bugs/124

It is necessary because coreutils uses glibc's regex.h,
which in turn uses `restrict'.  And that keyword isn't accepted
by gcc unless the --std=c99 option is used.

A language feature like `restrict' is fundamental enough that
autoconf should support it.  So, if no one objects, I'll add
AC_C_RESTRICT to autoconf.

Jim