configure.ac: add AM_PROG_CC_C_O

Older Automakes give this error without this directive:

Makefile.am: C objects in subdir but `AM_PROG_CC_C_O' not in `configure.ac'

In newer autotools this is included under AC_PROG_CC, but it's harmless
to add.

https://github.com/xkbcommon/libxkbcommon/issues/3

Signed-off-by: Ran Benita <ran234@gmail.com>