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/video/directfb/SDL_DirectFB_opengl.c

@@ -47,7 +47,7 @@ struct SDL_GLDriverData
 };
 
 #define OPENGL_REQUIRS_DLOPEN
-#if defined(OPENGL_REQUIRS_DLOPEN) && defined(SDL_LOADSO_DLOPEN)
+#if defined(OPENGL_REQUIRS_DLOPEN) && defined(HAVE_DLOPEN)
 #include <dlfcn.h>
 #define GL_LoadObject(X)    dlopen(X, (RTLD_NOW|RTLD_GLOBAL))
 #define GL_LoadFunction     dlsym

src/video/nacl/SDL_naclopengles.c

@@ -27,7 +27,7 @@
 #include "SDL_video.h"
 #include "SDL_naclvideo.h"
 
-#if SDL_LOADSO_DLOPEN
+#ifdef HAVE_DLOPEN
 #include "dlfcn.h"
 #endif
 
@@ -45,7 +45,7 @@ NACL_GLES_LoadLibrary(_THIS, const char *path)
 void *
 NACL_GLES_GetProcAddress(_THIS, const char *proc)
 {
-#if SDL_LOADSO_DLOPEN
+#ifdef HAVE_DLOPEN
     return dlsym( 0 /* RTLD_DEFAULT */, proc);
 #else
     return NULL;

src/video/x11/SDL_x11opengl.c

@@ -142,7 +142,7 @@ typedef GLXContext(*PFNGLXCREATECONTEXTATTRIBSARBPROC) (Display * dpy,
 #endif
 
 #define OPENGL_REQUIRES_DLOPEN
-#if defined(OPENGL_REQUIRES_DLOPEN) && defined(SDL_LOADSO_DLOPEN)
+#if defined(OPENGL_REQUIRES_DLOPEN) && defined(HAVE_DLOPEN)
 #include <dlfcn.h>
 #define GL_LoadObject(X)    dlopen(X, (RTLD_NOW|RTLD_GLOBAL))
 #define GL_LoadFunction     dlsym
@@ -174,7 +174,7 @@ X11_GL_LoadLibrary(_THIS, const char *path)
     }
     _this->gl_config.dll_handle = GL_LoadObject(path);
     if (!_this->gl_config.dll_handle) {
-#if defined(OPENGL_REQUIRES_DLOPEN) && defined(SDL_LOADSO_DLOPEN)
+#if defined(OPENGL_REQUIRES_DLOPEN) && defined(HAVE_DLOPEN)
         SDL_SetError("Failed loading %s: %s", path, dlerror());
 #endif
         return -1;