Further fixes and improvements related to WebAssembly

[oleg-derevenetz]

• Allow to build the Wasm version with or without multithreading support, as desired. The advantages of the non-multithreaded version are that it does not require a web server to return the special set of HTTP headers.
• Enable LTO for the Emscripten builds.
• Build the Emscripten version in the strict mode.
• Improve the mechanics of music feeding during playback, fix the deadlock (see the comments in the corresponding source files).

I'm currently running Wasm build of fheroes2 (with threads) in the browser (Chromium-based) in the AI auto control mode for a few hours, and I should say that the situation with playing background music has improved significantly. The only strange thing is that immediately after starting the game and when loading the first map, the memory consumption of the corresponding browser tab increases sharply from ~500Mb to 5-6Gb for several seconds ( and during this time music is played intermittently). After a few seconds, the memory consumption drops back to ~500Mb, and loading of other maps (or the same map) no longer causes such an effect, memory consumption is not growing anymore. This is very similar to the GC behavior, which means that some JavaScript code is most likely eating up memory (not the fheroes2 itself).

P.S. What's interesting, there is no such an effect in the Firefox.