Merged blaugold's WASM branch, plus fixes

[snej]

Cherry-picked @blaugold 's branch that adds WASM support via emscripten.
Made a few minor changes to get it to build again.
Added a Makefile to simplify (all types of) builds.

[borrrden]

Is this negated on purpose? Does ECMAScript somehow change the functionality here?

[blaugold]

It was necessary to make the tests pass, but it seems like a bug in Emscripten which implements all these UNIX APIs. Not sure what the current behavior is, though.

[borrrden]

Also none of this builds at all on Windows, is that known?

[borrrden]

It also appears to not build on Linux unless a specific version of emscripten is required? The final link fails possibly because of an unrecognized argument but somewhat unclear and too long to paste here.

With the latest emsdk it works on Ubuntu but the tests abort with the following:

node build_cmake/test_wasm/FleeceTests.js 7 ↵
failed to asynchronously prepare wasm: CompileError: WebAssembly.instantiate(): unexpected section (enable with --experimental-wasm-eh) @+20676
Aborted(CompileError: WebAssembly.instantiate(): unexpected section (enable with --experimental-wasm-eh) @+20676)
/home/borrrden/Development/couchbase-lite-c/vendor/couchbase-lite-core/vendor/fleece/build_cmake/test_wasm/FleeceTests.js:143
throw ex;
^
RuntimeError: Aborted(CompileError: WebAssembly.instantiate(): unexpected section (enable with --experimental-wasm-eh) @+20676)
at abort (/home/borrrden/Development/couchbase-lite-c/vendor/couchbase-lite-core/vendor/fleece/build_cmake/test_wasm/FleeceTests.js:697:41)
at /home/borrrden/Development/couchbase-lite-c/vendor/couchbase-lite-core/vendor/fleece/build_cmake/test_wasm/FleeceTests.js:765:5