Make the Dynamic API master switch more clear.

src/dynapi/SDL_dynapi.h

@@ -49,7 +49,10 @@
 #define SDL_DYNAMIC_API 0
 #elif defined(__clang_analyzer__)
 #define SDL_DYNAMIC_API 0  /* Turn off for static analysis, so reports are more clear. */
-#else   /* everyone else. */
+#endif
+
+/* everyone else. This is where we turn on the API if nothing forced it off. */
+#ifndef SDL_DYNAMIC_API
 #define SDL_DYNAMIC_API 1
 #endif