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>

configure.ac

@@ -49,6 +49,7 @@ dnl Check for progs
 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