Enable pthreads in libc++ for THREAD_MODEL=single.

This commit enables thread support in libc++ for all thread models,
enabling C++ applications that use threading APIs like `<atomic>` but
do not spawn threads (e.g. Clang) to be built with minimal changes.

Fixes #546.

Author: whitequark

README.md

@@ -214,10 +214,21 @@ See [C setjmp/longjmp support] about setjmp/longjmp support.
 [C setjmp/longjmp support]: SetjmpLongjmp.md
 
 This repository experimentally supports __threads__ with
-`--target=wasm32-wasi-threads`. It uses WebAssembly's [threads] primitives
+`--target=wasm32-wasip1-threads`. It uses WebAssembly's [threads] primitives
 (atomics, `wait`/`notify`, shared memory) and [wasi-threads] for spawning
 threads. Note: this is experimental — do not expect long-term stability!
 
+Note that the `pthread_*` family of functions, as well as C++ threading primitives
+such as `<atomic>`, `<mutex>`, and `<thread>` are available on all targets.
+Any attempt to spawn a thread will fail on `--target=wasm32-wasip1` or
+`--target=wasm32-wasip2`, but other functionality, such as locks, still works.
+This makes it easier to port C++ codebases, as only a fraction of code needs
+to be modified to build for the single-threaded targets.
+
+Defining a macro `_WASI_STRICT_PTHREAD` will make `pthread_create`,
+`pthread_detach`, `pthread_join`, `pthread_tryjoin_np`, and `pthread_timedjoin_np`
+fail with a compile time error when building for single-threaded targets.
+
 [threads]: https://github.com/WebAssembly/threads
 [wasi-threads]: https://github.com/WebAssembly/wasi-threads
 

cmake/wasi-sdk-sysroot.cmake

@@ -226,7 +226,7 @@ function(define_libcxx_sub target target_suffix extra_target_flags extra_libdir_
     set(pic OFF)
     set(target_flags -pthread)
   else()
-    set(threads OFF)
+    set(threads ON) # uses the thread-stub implementation
     set(pic ON)
     set(target_flags "")
   endif()