Use the correct macro to make automake happy.

Reported by <bsdfan666@cyberdude.com>.

configure.ac

@@ -5,7 +5,7 @@ AM_INIT_AUTOMAKE([foreign subdir-objects])
 AC_CONFIG_FILES([Makefile])
 
 AC_PROG_CC
-AC_PROG_CC_C_O
+AM_PROG_CC_C_O
 AC_PROG_RANLIB
 AC_USE_SYSTEM_EXTENSIONS