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

CMakeLists.txt

@@ -303,13 +303,11 @@ if(EMSCRIPTEN)
   set(SDL_ATOMIC_ENABLED_BY_DEFAULT OFF)
   set(SDL_LOADSO_ENABLED_BY_DEFAULT OFF)
   set(SDL_CPUINFO_ENABLED_BY_DEFAULT OFF)
-  set(SDL_DLOPEN_ENABLED_BY_DEFAULT OFF)
 endif()
 
 if(VITA)
   set(SDL_SHARED_ENABLED_BY_DEFAULT OFF)
   set(SDL_LOADSO_ENABLED_BY_DEFAULT OFF)
-  set(SDL_DLOPEN_ENABLED_BY_DEFAULT OFF)
 endif()
 
 # When defined, respect CMake's BUILD_SHARED_LIBS setting:
@@ -365,7 +363,6 @@ set_option(VIDEO_OPENGL        "Include OpenGL support" ON)
 set_option(VIDEO_OPENGLES      "Include OpenGL ES support" ON)
 set_option(PTHREADS            "Use POSIX threads for multi-threading" ${SDL_PTHREADS_ENABLED_BY_DEFAULT})
 dep_option(PTHREADS_SEM        "Use pthread semaphores" ON "PTHREADS" OFF)
-set_option(SDL_DLOPEN          "Use dlopen for shared object loading" ${SDL_DLOPEN_ENABLED_BY_DEFAULT})
 dep_option(OSS                 "Support the OSS audio API" ON "UNIX_SYS OR RISCOS" OFF)
 set_option(ALSA                "Support the ALSA audio API" ${UNIX_SYS})
 dep_option(ALSA_SHARED         "Dynamically load ALSA audio support" ON "ALSA" OFF)
@@ -926,11 +923,6 @@ if(SDL_HAPTIC)
 endif()
 
 
-# TODO: in configure.ac, the test for LOADSO and SDL_DLOPEN is a bit weird:
-# if LOADSO is not wanted, SDL_LOADSO_DISABLED is set
-# If however on Unix or APPLE dlopen() is detected via CheckDLOPEN(),
-# SDL_LOADSO_DISABLED will not be set, regardless of the LOADSO settings
-
 # General SDL subsystem options, valid for all platforms
 if(SDL_AUDIO)
   # CheckDummyAudio/CheckDiskAudio - valid for all platforms
@@ -948,10 +940,15 @@ if(SDL_AUDIO)
   endif()
 endif()
 
-if(SDL_DLOPEN)
+if(UNIX OR APPLE)
   # Relevant for Unix/Darwin only
-  if(UNIX OR APPLE)
-    CheckDLOPEN()
+  set(DYNAPI_NEEDS_DLOPEN 1)
+  CheckDLOPEN()
+  if(SDL_LOADSO AND HAVE_DLOPEN)
+    set(SDL_LOADSO_DLOPEN 1)
+    file(GLOB DLOPEN_SOURCES ${SDL2_SOURCE_DIR}/src/loadso/dlopen/*.c)
+    set(SOURCE_FILES ${SOURCE_FILES} ${DLOPEN_SOURCES})
+    set(HAVE_SDL_LOADSO TRUE)
   endif()
 endif()