cmake: detect RISCOS platform before GNU/Hurd

cmake/sdlplatform.cmake

@@ -26,6 +26,8 @@ function(SDL_DetectCMakePlatform)
     set(sdl_cmake_platform ngage)
   elseif(PS2)
     set(sdl_cmake_platform ps2)
+  elseif(RISCOS)
+    set(sdl_cmake_platform RISCOS)
   elseif(VITA)
     set(sdl_cmake_platform Vita)
   elseif(CMAKE_SYSTEM_NAME MATCHES ".*Linux")
@@ -37,6 +39,7 @@ function(SDL_DetectCMakePlatform)
   elseif(CMAKE_SYSTEM_NAME MATCHES "kOpenBSD.*|OpenBSD.*")
     set(sdl_cmake_platform OpenBSD)
   elseif(CMAKE_SYSTEM_NAME STREQUAL "GNU")
+    # GNU/Hurd must be checked AFTER RISCOS
     set(sdl_cmake_platform Hurd)
   elseif(CMAKE_SYSTEM_NAME MATCHES ".*BSDI.*")
     set(sdl_cmake_platform BSDi)