Add -D_DEBUG when --enable-debug is used, this can't be turned off as

anyone making a debug build should use any features to help out with
debugging.

Originally committed to SVN as r2737.

configure.in

@@ -1055,6 +1055,8 @@ if test "$enable_debug" = "yes"; then
   CPPFLAGS="$DEBUG_FLAGS $CPPFLAGS"
   msg_debug="($DEBUG_FLAGS)"
   AC_SUBST(DEBUG_FLAGS)
+  # This turns on some internal (to aegisub) debugging features.
+  AC_CXX_FLAG([-D_DEBUG])
 else
   enable_debug="no"
   AC_MSG_RESULT([no])