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>
diff --git a/configure.ac b/configure.ac
index df65ae5..9d6cbeb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -49,6 +49,7 @@
 AC_PROG_CPP
 AC_PROG_CC
 AC_PROG_CXX
+AM_PROG_CC_C_O
 AC_CHECK_PROGS([MAKE], [gmake make])
 AC_CHECK_PROGS([PYTHON2], [python2 python])
 AC_PROG_SED