autoconf: add AM_PROG_CC_C_O

Prevents this error with Automake 1.9:

src/gallium/drivers/Makefile.am: C objects in subdir but
`AM_PROG_CC_C_O' not in `configure.ac'
autoreconf: automake failed with exit status: 1

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
1 file changed