AC_PROG_CC_C89, AC_PROG_CC_C99: now obsolete; defer to AC_PROG_CC

AC_PROG_CC_C89, AC_PROG_CC_C99: now obsolete; defer to AC_PROG_CC

* NEWS:
* doc/autoconf.texi (C Compiler, Running the Preprocessor)
(Limitations of Usual Tools, Present But Cannot Be Compiled)
(Obsolete Macros):
Document the changes described below.
* lib/autoconf/c.m4 (_AC_PROG_CC_FORCE_VERSION): Remove.
(AC_PROG_CC_C89, AC_PROG_CC_C99, AC_PROG_CC_STDC):
Just do AC_PROG_CC, but mark as obsolete.  This replaces my recent
ill-advised attempt to let AC_PROG_CC_C89 and AC_PROG_CC_C99 downgrade
the version of C supported.
* doc/autoconf.texi (Limitations of Usual Tools, Volatile Objects):
Document C11 more accurately.  In some cases this involves removing
some details about 'volatile', alas, since C11 changed this stuff.
Again.