Fixed the build for web using CMake.

[object71]

I found that the build for me was failing and I added some if defined checks in the core.c file where the glfwSetWindowAttrib was used. (error: implicit declaration of function 'glfwSetWindowAttrib' is invalid in C99 [-Werror,-Wimplicit-function-declaration])

I also changed some values in the toolchain file so that it correctly uses the .bat files when on windows.

[object71]

@raysan5 Hi, I made some improvements on the CMakeLists.txt files. The project wasn't build-able for me for web on windows and I decided to improve some of the cmake configurations.

I edited some values in the cmake/emscripten.cmake toolchain that is local in the project as on windows it didn't correctly execute the .bat compilers.

I added the option to detect EMSCRIPTEN variable set by the emscripten toolchain. The emscripten toolchain is set by default when the emcmake is used before cmake.

I fixed an issue with a replace value by putting quotes around it (the value was empty for some reason for me)

I fixed the extension filename generation for web to properly generate .html files.

Now the process for build web is more straighforward - open terminal in project root and generate with "emcmake cmake -S . -B build -G Ninja"

[raysan5]

@object71 Nice! Thanks for the improvements! I don't maintain the CMake build system... Maybe @a3f could take a look to them?

[a3f]

CMake parts look ok to me.

[raysan5]

@a3f Nice! Thanks for the review!

[object71]

@raysan5 Thanks, I'll see to update the documentation on building for web using cmake now.