cmake: Report enabled subsystem backends in a compact form. (#13926)

Looks like this:

```
-- Enabled backends:
--   Video drivers: dummy kmsdrm(dynamic) offscreen wayland(dynamic) x11(dynamic)
--   X11 libraries: xcursor xdbe xfixes xinput2 xrandr xscrnsaver xshape xsync xtest
--   Render drivers: gpu ogl vulkan
--   GPU drivers: vulkan
--   Audio drivers: alsa(dynamic) disk dummy jack(dynamic) pipewire(dynamic) pulseaudio(dynamic) sndio(dynamic)
--   Joystick drivers: hidapi linux virtual
```

Fixes #7922.

Co-authored-by: Anonymous Maarten <[email protected]>

cmake/macros.cmake

@@ -334,6 +334,29 @@ if(APPLE)
   endif()
 endif()
 
+function(PrintEnabledBackends _SUBSYS _REGEXP)
+  get_cmake_property(_ALLVARS VARIABLES)
+  foreach(_VAR IN LISTS _ALLVARS)
+    if(_VAR AND _VAR MATCHES "${_REGEXP}")
+      string(TOLOWER "${CMAKE_MATCH_1}" _LOWERED)
+      if(NOT _LOWERED MATCHES "available|default|dynamic")  # a little hack
+        if(${_VAR}_DYNAMIC)
+          list(APPEND _ENABLED_BACKENDS "${_LOWERED}(dynamic)")
+        else()
+          list(APPEND _ENABLED_BACKENDS "${_LOWERED}")
+        endif()
+      endif()
+    endif()
+  endforeach()
+
+  if(_ENABLED_BACKENDS STREQUAL "")
+    set(_SPACEDOUT "(none)")
+  else()
+    string(REPLACE ";" " " _SPACEDOUT "${_ENABLED_BACKENDS}")
+  endif()
+  message(STATUS "  ${_SUBSYS}: ${_SPACEDOUT}")
+endfunction()
+
 function(SDL_PrintSummary)
   ##### Info output #####
   message(STATUS "")
@@ -366,6 +389,18 @@ function(SDL_PrintSummary)
     message(STATUS " Build libraries as Apple Framework: ${SDL_FRAMEWORK}")
   endif()
   message(STATUS "")
+
+  message(STATUS "Enabled backends:")
+  PrintEnabledBackends("Video drivers" "^SDL_VIDEO_DRIVER_([A-Z0-9]*)$")
+  if(SDL_VIDEO_DRIVER_X11)
+    PrintEnabledBackends("X11 libraries" "^SDL_VIDEO_DRIVER_X11_([A-Z0-9]*)$")
+  endif()
+  PrintEnabledBackends("Render drivers" "^SDL_VIDEO_RENDER_([A-Z0-9]*)$")
+  PrintEnabledBackends("GPU drivers" "^SDL_GPU_([A-Z0-9]*)$")
+  PrintEnabledBackends("Audio drivers" "^SDL_AUDIO_DRIVER_([A-Z0-9]*)$")
+  PrintEnabledBackends("Joystick drivers" "^SDL_JOYSTICK_([A-Z0-9]*)$")
+  message(STATUS "")
+
   if(UNIX)
     message(STATUS "If something was not detected, although the libraries")
     message(STATUS "were installed, then make sure you have set the")