configure/cmake: Hook up Emscripten threads (disabled by default).

Fixes #3795.

cmake/sdlchecks.cmake

@@ -919,6 +919,9 @@ macro(CheckPTHREAD)
     elseif(HAIKU)
       set(PTHREAD_CFLAGS "-D_REENTRANT")
       set(PTHREAD_LDFLAGS "")
+    elseif(EMSCRIPTEN)
+      set(PTHREAD_CFLAGS "-D_REENTRANT -pthread")
+      set(PTHREAD_LDFLAGS "-pthread")
     else()
       set(PTHREAD_CFLAGS "-D_REENTRANT")
       set(PTHREAD_LDFLAGS "-lpthread")