I 'extracted' the source code for godot shader transcompiler from Godot into this simple library that converts Godot's shaders into GLSL. It is lightweight and has no dependencies.
It is super easy to use:
#include <GodotShaderTranscompiler/ShaderTranscompiler.h>
...
gd::GLSLOutput out;
gd::ShaderTranscompiler::Transcompile("GODOT SHADER HERE", out);
...GLSLOutput structure will contain various information about the godot shader you've compiled, including the GLSL source code for vertex and fragment shaders.