Added a warning comment to SDL_putenv().

"Fixes" Bugzilla #779.

include/SDL_stdinc.h

@@ -267,6 +267,16 @@ char *alloca();
 extern DECLSPEC char *SDLCALL SDL_getenv(const char *name);
 #endif
 
+/**
+ * \warning On some platforms, the string you pass to SDL_putenv() becomes
+ *          part of the environment table directly...it will use this specific
+ *          buffer, and not a copy of it! This means you can't free it, and
+ *          other pieces of code may try to write to it. In practice, this
+ *          isn't a big deal, but be aware of the possibility.
+ *          However, due to this issue, you should be prepared to
+ *          pass a (char*), or be willing to cast away the constness of your
+ *          string for this call.
+ */
 #ifdef HAVE_PUTENV
 #define SDL_putenv	putenv
 #else