Bug: Pthreads in Assimp 5.3.0 with vcpkg

**Describe the bug**
I want to compile a C++ Cmake-project on Windows with emscripten to WebAssembly. I was able to successfully use the Qt 6.6.2 WebAssembly (single-threaded) Kit. Therefore i installed zlib and assimp via vcpkg and emcmdprompt and also added the toolchain like this:
`set(CMAKE_TOOLCHAIN_FILE "C:...\\vcpkg\\scripts\\buildsystems\\vcpkg.cmake")` in my Cmake-File right at the top
I also set the `VCPKG_CHAINLOAD_TOOLCHAIN_FILE` in the initial configuration to the Path

> C:\Qt\6.6.2\wasm_singlethread\lib\cmake\Qt6\qt.toolchain.cmake

As soon as I switch from the single-threaded to the multi-threaded kit and change the VCPKG_CHAINLOAD_TOOLCHAIN_FILE accordingly to:

> C:\Qt\6.6.2\wasm_multithread\lib\cmake\Qt6\qt.toolchain.cmake

I get the following ninja build error when trying to build
`wasm-ld: error: --shared-memory is disallowed by 3DSLoader.cpp.o because it was not compiled with 'atomics' or 'bulk-memory' features`

The problem seems to be that the pthreads option does not seem to be enabled on assimp. I tried to check if there was such an option on installation with vcpkg in

> \\vcpkg\\buildtrees\\assimp\\src\\v5.3.1-c1ad782f20.clean\\Build.md

I need to mention that I am fairly new to everything on this topic, so feel free to correct me on anything i might have done wrong.

**To Reproduce**
Steps to reproduce the behavior:
I have installed the following programs/packages:
- CMake 3.29.0
- Python 3.12.2 (64-bit)
- Qt Creator 12.0.2 with Qt 6.6.2
- emscripten package 3.1.37
- vcpkg for windows
- zlib and assimp via emcmdprompt and wasm32-emscripten.cmake triplet

Then i set the Variables and included assimp in Cmake.

**Expected behavior**
The Qt 6.6.2 multithreading Kit (with emsdk 3.1.37) compiles the assimp Library with pthreads enabled.
**Screenshots**

**Platform (please complete the following information):**
- OS: Windows 10
- Browser Google Chrome Version 123.0.6312.106 (64-Bit)

**Additional information**
This is CmakeLists.txt where i see the most potential in fixing or me being the problem.
I one tried ther set(WASM_BUILD_FLAGS) but could not find any proper documentation on using it like here?
https://github.com/emscripten-core/emscripten/issues/20129
I also tried the -pthread option on target_compile_options.

```
set(CMAKE_TOOLCHAIN_FILE "C:\\MA_Clothing_Simulator\\ClothSimulator\\Packages\\vcpkg\\scripts\\buildsystems\\vcpkg.cmake")

cmake_minimum_required(VERSION 3.5)

project(Clothing-Simulator VERSION 0.1 LANGUAGES CXX)

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(FORCE_EXCEPTION_FLAG "-fwasm-exceptions")

find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Core Gui OpenGL OpenGLWidgets Widgets)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Widgets)
find_package(assimp 5.3.0 CONFIG REQUIRED)

set(PROJECT_SOURCES
main.cpp
)

if(${QT_VERSION_MAJOR} GREATER_EQUAL 6)
qt_add_executable(Clothing-Simulator
MANUAL_FINALIZATION
${PROJECT_SOURCES}
model.h model.cpp
view.h view.cpp
controller.h controller.cpp
physicsengine.h physicsengine.cpp
mesh.h mesh.cpp
stb_image.h
ipublisher.h
iobserver.h
mvcfactory.h
)

target_compile_options(Clothing-Simulator PRIVATE -Wextra -Wall -Wsign-conversion -Wfloat-equal -pedantic -Wredundant-decls -Wshadow -Wpointer-arith -O3 ${FORCE_EXCEPTION_FLAG})

set_target_properties(Clothing-Simulator PROPERTIES
${BUNDLE_ID_OPTION}
MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}
MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
MACOSX_BUNDLE TRUE
WIN32_EXECUTABLE TRUE
)

target_link_libraries(Clothing-Simulator PRIVATE
Qt6::Core
Qt6::Gui
Qt6::OpenGL
Qt6::OpenGLWidgets
Qt6::Widgets
assimp::assimp
)

# Objects:
set(object_resource_files
"assets/Exocube/Exocube.obj"
)

qt6_add_resources(Clothing-Simulator "objects"
PREFIX
"/assets/"
FILES
${object_resource_files}
)

target_link_options(Clothing-Simulator PUBLIC --embed-file --bind
${CMAKE_SOURCE_DIR}/assets@/
${FORCE_EXCEPTION_FLAG}
)

target_include_directories(Clothing-Simulator PUBLIC ${CMAKE_CURRENT_LIST_DIR})

# Resources:
set(shaders_resource_files
"fshader.glsl"
"vshader.glsl"
)

qt6_add_resources(Clothing-Simulator "shaders"
PREFIX
"/"
FILES
${shaders_resource_files}
)

set(textures_resource_files
"test.png"
"assets/Exocube/texture_diffuse.png"
)

qt6_add_resources(Clothing-Simulator "textures"
PREFIX
"/"
FILES
${textures_resource_files}
)

//Define target properties for Android with Qt 6 as: set_property(TARGET Clothing-Simulator APPEND PROPERTY //QT_ANDROID_PACKAGE_SOURCE_DIR
// ${CMAKE_CURRENT_SOURCE_DIR}/android)
// For more information, see https://doc.qt.io/qt-6/qt-add-executable.html#target-creation
else()
if(ANDROID)
add_library(Clothing-Simulator SHARED
${PROJECT_SOURCES}
)
// Define properties for Android with Qt 5 after find_package() calls as:
// set(ANDROID_PACKAGE_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/android")
else()
add_executable(Clothing-Simulator
${PROJECT_SOURCES}
)
endif()
endif()

include(GNUInstallDirs)
install(TARGETS Clothing-Simulator
BUNDLE DESTINATION .
LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
)

if(QT_VERSION_MAJOR EQUAL 6)
qt_finalize_executable(Clothing-Simulator)
endif()
```

Start with the provided CMakeLists.txt and the assimp Build.md under vcpkg/buildtrees/assimp, then inspect the generated compile and link flags for 3DSLoader.cpp.o in single- and multi-threaded Emscripten builds. Confirm whether assimp receives the required pthread-related flags; done means a documented, reproducible configuration that builds assimp for the Qt multithreaded WebAssembly kit or clearly identifies the missing integration.

Written by the indexing model from the issue text.