Main project page: http://emscripten.org
Emscripten provides Web support for popular portable APIs such as OpenGL and SDL2, allowing complex graphical native applications to be ported, such as the Unity game engine and Google Earth. It can probably port your codebase, too!
While Emscripten mostly focuses on compiling C and C++ using Clang, it can be integrated with other LLVM-using compilers (for example, Rust has Emscripten integration, with the
Emscripten is available under 2 licenses, the MIT license and the University of Illinois/NCSA Open Source License.
Both are permissive open source licenses, with little if any practical difference between them.
The reason for offering both is that (1) the MIT license is well-known, while (2) the University of Illinois/NCSA Open Source License allows Emscripten's code to be integrated upstream into LLVM, which uses that license, should the opportunity arise.
LICENSE for the full content of the licenses.