cmake: exclude -pthread from Emscripten default build

cmake/OpenCVCompilerOptions.cmake

@@ -177,7 +177,13 @@ if(CV_GCC OR CV_CLANG)
   endif()
 
   # We need pthread's
-  if(UNIX AND NOT ANDROID AND NOT (APPLE AND CV_CLANG)) # TODO
+  if((UNIX
+          AND NOT ANDROID
+          AND NOT (APPLE AND CV_CLANG)
+          AND NOT EMSCRIPTEN
+      )
+      OR (EMSCRIPTEN AND WITH_PTHREADS_PF)  # https://github.com/opencv/opencv/issues/20285
+  )
     add_extra_compiler_option(-pthread)
   endif()