Configure SDL assuming we, in fact, have a libc.

Without this, on Linux and Linux alone, and only if SDL is builtin, the
definitions for some functions - notably, memset/memcpy - are taken from
SDL source.

This leaves them at the mercy of the compiler and may lead to the
optimal implementations of these functions not being selected. But the
main motivation for this is that it's action at a distance that is
incredibly surprising - linking in a gamepad support library should not
change the implementation of memset/memcpy!

drivers/sdl/SDL_build_config_private.h

@@ -81,6 +81,7 @@
 #define SDL_PLATFORM_UNIX 1
 
 #define HAVE_STDIO_H 1
+#define HAVE_LIBC 1
 #define HAVE_LINUX_INPUT_H 1
 #define HAVE_POLL 1