BOOST_HAS_PTHREADS not set with emscripten

This looks like a regression on #402

Minimal example to reproduce with cmake:

```cmake

cmake_minimum_required(VERSION 3.27)
project(boost_wasm_test)

set(CMAKE_CXX_STANDARD 20)

include(FetchContent)

FetchContent_Declare(
Boost
URL https://github.com/boostorg/boost/releases/download/boost-1.87.0/boost-1.87.0-cmake.7z
EXCLUDE_FROM_ALL
)
set(BOOST_INCLUDE_LIBRARIES geometry)
FetchContent_MakeAvailable(Boost)

add_executable(boost_wasm_test main.cpp)
target_link_libraries(boost_wasm_test Boost::geometry)
target_compile_options(boost_wasm_test PRIVATE -pthread)
target_link_options(boost_wasm_test PRIVATE -pthread)

```

(main.cpp is just generic hello world)

I get lots of errors like

```
In file included from /home/m/Documents/Linos/boost_wasm_test/build_emscripten/_deps/boost-src/libs/container/src/synchronized_pool_resource.cpp:14:
/home/m/Documents/Linos/boost_wasm_test/build_emscripten/_deps/boost-src/libs/container/include/boost/container/detail/thread_mutex.hpp:109:12: error: '__declspec' attributes are not enabled; use '-fdeclspec' or '-fms-extensions' to enable support for __declspec attributes
109 | extern "C" __declspec(dllimport) void __stdcall InitializeCriticalSection(::_RTL_CRITICAL_SECTION *);
```

Which indicates, that BOOST_HAS_PTHREADS is not properly set.
Or am I using CMake wrong?

I am on emscripten v3.1.61
Same result with latest emscripten 4.04