CMake: Fix missing -sUSE_PTHREADS=1 in web build

[enetheru]

@lechaosx identified an issue with compiling for web using cmake in issue #1830

This led me to discover that there was a mising flag (-sUSE_PTHREADS=1) during the link stage that prevented godot from loading the library if threads were enabled.

This PR adds the missing flag, and adds a tiny change which brings it inline with scons.

• add -sUSE_PTHREADS=1 to link flags
• add =1 to sSIDE_MODULE=1 in compile flags to match scons.

The other proposed changes to cache the values of the emsdkHack.cmake are not going to solve the inclusion problem, as the special case for emscripten is that the values need to be injected as part of the project instantiation. This unfortunatley makes it that any project added for emscripten needs the include injection variable set prior to its definion.

set(CMAKE_PROJECT_<my project name goes here>_INCLUDE ${godot-cpp_SOURCE_DIR}/cmake/emsdkHack.cmake)

[dsnopek]

Thanks!

These changes make sense to me. I tested, and the command line shown via --verbose looks good.

However, could you change the commit message to specifically refer to what this does rather than the Godot issue? Like, "CMake: Fix missing -sUSE_PTHREADS=1 in web build" or something like that

This unfortunatley makes it that any project added for emscripten needs the include injection variable set prior to its definion.

set(CMAKE_PROJECT_<my project name goes here>_INCLUDE ${godot-cpp_SOURCE_DIR}/cmake/emsdkHack.cmake)

So, are you saying this is something we'll need to document that developers need to add to their projects?

[enetheru]

updated the commit message.

So, are you saying this is something we'll need to document that developers need to add to their projects?

Yes, this is the case, and it's entirely unavoidable due to the way cmake works. If it were something I could fix I would eliminate it as a problem. I feel like a guide for consumers is necessary at this point, though I don't know where I would put it.

[dsnopek]

Thanks!

I feel like a guide for consumers is necessary at this point, though I don't know where I would put it.

We have a section in the docs for Emscripten with CMake - we could document it there?

[enetheru]

We have a section in the docs for Emscripten with CMake - we could document it there?

Yeah I guess, that whole document is written from the perspective of us godot-cpp devs compiling godot-cpp, not a consumer setting up cmake for thier project, hence my hesitation.

[dsnopek]

Yeah I guess, that whole document is written from the perspective of us godot-cpp devs compiling godot-cpp, not a consumer setting up cmake for thier project, hence my hesitation.

Hm, I always thought of that documentation as being for consumers using the CMake configuration in their project, with a few notes for godot-cpp contributors (for example, how to build the tests). We should probably revamp it to actually be that!