Fix mj_loadXML hang in single-threaded WASM builds

The top-level CMakeLists.txt unconditionally added -pthread for all Emscripten builds, but PTHREAD_POOL_SIZE was only set when MUJOCO_WASM_THREADS=ON. This gave the ST build a live pthread runtime with zero pre-spawned workers, causing CompileMeshesAndTextures to deadlock when it tried to spawn a ThreadPool on the main thread.

Fix by making -pthread conditional on MUJOCO_WASM_THREADS. Without -pthread, hardware_concurrency() returns 0 and the existing nthread < 2 guards naturally select the serial compilation path.

PiperOrigin-RevId: 900191420
Change-Id: I0481b81fc6f7cccb52c7d7592dcf9dd317b5bad1

CMakeLists.txt

@@ -53,8 +53,12 @@ endif()
 
 if(EMSCRIPTEN)
   option(MUJOCO_BUILD_TESTS_WASM "Build tests for WASM bindings" ON)
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++20 -O3 -pthread -fexceptions")
+  option(MUJOCO_WASM_THREADS "Build with multi-threading support" ON)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++20 -O3 -fexceptions")
+  if(MUJOCO_WASM_THREADS)
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
+  endif()
 endif()
 
 if(APPLE AND (MUJOCO_BUILD_EXAMPLES OR MUJOCO_BUILD_SIMULATE))

wasm/CMakeLists.txt

@@ -31,8 +31,6 @@ if(NOT MUJOCO_WASM_FILES)
     message(FATAL_ERROR "No source files found in codegen/generated/")
 endif()
 
-option(MUJOCO_WASM_THREADS "Build with multi-threading support" ON)
-
 # Set Emscripten linker flags
 set(EMCC_LINKER_FLAGS
     "--bind"