Avoid that ruby 1.9 redefines snprintf(3) etc.

Otherwise our custom printf specifiers won't work.

src/dumm/ext/dumm.c

@@ -30,6 +30,8 @@
 #undef PACKAGE_STRING
 #undef PACKAGE_BUGREPORT
 #undef PACKAGE_URL
+/* avoid redefintiion of snprintf etc. */
+#define RUBY_DONT_SUBST
 #include <ruby.h>
 
 static dumm_t *dumm;