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>

configure.ac

@@ -49,6 +49,9 @@ XORG_MEMORY_CHECK_FLAGS
 XORG_ENABLE_DOCS
 XORG_WITH_DOXYGEN
 
+# Needed in older Automakes for subdir-objects
+AM_PROG_CC_C_O
+
 # Check for compiler features
 AC_C_INLINE