Support shared libraries "properly"

[dneto0]

See also KhronosGroup/SPIRV-Tools#3214

[BurningEnlightenment]

As a bystander who just had to rewrite the shaderc vcpkg port I can say that this would be a nice thing to have.
Additionally I recommend bumping the required cmake version to something >= 3.5 and refactor/modernize the cmake buildfiles to fully utilize the power of cmake targets. Compiler flag propagation through target dependency chains could remove a lot of the repeatedly specified settings like include directories, link dependencies, preprocessor definitions, etc. It would also allow for proper target install configs which in turn would allow find_package(shaderc) to just work™️. Bonus points if find_package(glslang CONFIG) & find_package(SPIRV-Tools CONFIG) get asked allowing the user to easily provide the dependencies himself.
If the modern cmake approach is deemed worth a try, feel free to ping me in case you encounter obstacles/unclear stuff. I have spend quite some time writing cmake buildsystems and wouldn't mind sharing some of the lessons I learned.

However, thanks for all the hard work!