The problem isn't just compiling C++ to glsl. The problem is taking c++ and allowing you to write your shader as part of your program. Right now, you essentially have 2 options: you can write one shader that does everything and use the actual data passed to it to conditionally run what you want, or you build a shader composition system that takes small chunks of code and pieces them together based on what data you want to pass to it. Thats overly simplified, but a kind of high level view. There are a few talks from bungie detailing the work they put into their shader system so that technical artists could have a smooth workflow.
Ultimately, your engine can't handle custom shaders without the engine user also writing code for the cpu side, unless you have a AAA sized engine team that develops a compiler to achieve that.