cmake: Preseed CMake cache for Emscripten builds.

Makes a from-scratch build on this laptop go from 2 minutes 42 seconds to
28 seconds!

CMakeLists.txt

@@ -52,10 +52,14 @@ include("${SDL3_SOURCE_DIR}/cmake/sdltargets.cmake")
 include("${SDL3_SOURCE_DIR}/cmake/GetGitRevisionDescription.cmake")
 include("${SDL3_SOURCE_DIR}/cmake/3rdparty.cmake")
 include("${SDL3_SOURCE_DIR}/cmake/PreseedMSVCCache.cmake")
+include("${SDL3_SOURCE_DIR}/cmake/PreseedEmscriptenCache.cmake")
 
 SDL_DetectCompiler()
 SDL_DetectTargetCPUArchitectures(SDL_CPUS)
-SDL_Preseed_CMakeCache()
+
+if (SDL_PRESEED_FUNCTION_DEFINED)
+  SDL_Preseed_CMakeCache()
+endif()
 
 # Increment this if there is an incompatible change - but if that happens,
 # we should rename the library from SDL3 to SDL4, at which point this would