configure.ac: drop macro 'AC_PROG_CC_C99' from autoconf 2.70

configure.ac: drop macro 'AC_PROG_CC_C99' from autoconf 2.70

The macro AC_PROG_CC_C99 has been obsoleted by autoconf 2.70 and
triggers a set of warnings from both 'aclocal' and 'autoconf'.
The test of AC_PROG_CC_C99 is now included in AC_PROG_CC.

For autoconf 2.69 and earlier the macro is still required, so
cannot be simply dropped.

Use a conditional test to avoid the warning on autoconf 2.70 but
still use AC_PROG_CC_C99 on older autoconf.

Change-Id: I5e8437f5a826fb63be6d07bcb5bb824f94683020
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6009
Tested-by: jenkins
Reviewed-by: Marc Schink <dev@zapb.de>