configuration updates for dlopen:

- cmake, configure (CheckDLOPEN): --enable-sdl-dlopen is now history..
  detach the dl api discovery from SDL_LOADSO_DLOPEN functionality.
  define HAVE_DLOPEN. also define DYNAPI_NEEDS_DLOPEN (CheckDLOPEN is
  called only for relevant platforms.)
- update SDL_config.in and SDL_config.cmake accordingly.
- SDL_dynapi.h: set SDL_DYNAMIC_API to 0 if DYNAPI_NEEDS_DLOPEN is
  defined, but HAVE_DLOPEN is not.
- pthread/SDL_systhread.c: conditionalize dl api use to HAVE_DLOPEN
- SDL_x11opengl.c, SDL_DirectFB_opengl.c, SDL_naclopengles.c: rely
  on HAVE_DLOPEN, not SDL_LOADSO_DLOPEN.
- SDL_config_android.h, SDL_config_iphoneos.h, SDL_config_macosx.h,
  SDL_config_pandora.h, and SDL_config_wiz.h: define HAVE_DLOPEN.

Closes: https://github.com/libsdl-org/SDL/pull/4351

src/dynapi/SDL_dynapi.h

@@ -59,6 +59,8 @@
 #define SDL_DYNAMIC_API 0  /* Turn off for static analysis, so reports are more clear. */
 #elif defined(__VITA__)
 #define SDL_DYNAMIC_API 0  /* vitasdk doesn't support dynamic linking */
+#elif defined(DYNAPI_NEEDS_DLOPEN) && !defined(HAVE_DLOPEN)
+#define SDL_DYNAMIC_API 0  /* we need dlopen(), but don't have it.... */
 #endif
 
 /* everyone else. This is where we turn on the API if nothing forced it off. */