Make sure gcc gives error for passing scoped_dohptr to Printv()

After making this mistake too many times already, make sure it doesn't
happen again by requiring gcc to give an error for any attempt to pass
a non-trivial object to vararg functions such as Printv(), Printf(),
Swig_error() etc.

Source/Modules/c.cxx

@@ -29,6 +29,13 @@ int SwigType_isbuiltin(SwigType *t) {
 namespace
 {
 
+// When using scoped_dohptr, it's very simple to accidentally pass it to a vararg function, such as Printv() or Printf(), resulting in catastrophic results
+// during run-time (crash or, worse, junk in the generated output), so make sure gcc warning about this, which is not enabled by default for some reason (see
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64867 for more information), is enabled.
+#ifdef __GNUC__
+  #pragma GCC diagnostic error "-Wconditionally-supported"
+#endif // __GNUC__
+
 // Delete a DOH object on scope exit.
 class scoped_dohptr
 {