Namespace CMake build options

Glslang now uses very generic names for CMake options such as `BUILD_SHARED_LIBS`. This may cause conflicts/confusion when integrating glslang in larger projects (https://github.com/DiligentGraphics/DiligentEngine/issues/45).
It is preferred to namespace global settings such as `GLSLANG_BUILD_SHARED_LIBS` (similar to for instance how [SPIRV-Tools](https://github.com/KhronosGroup/SPIRV-Tools/blob/master/CMakeLists.txt#L66) does that) to clearly separate them.

Start by inspecting glslang's CMake configuration and the global options it defines, then compare the naming approach in the referenced SPIRV-Tools CMakeLists.txt. Namespace the generic settings consistently and verify that the project still configures and builds when integrated into a larger CMake project.

Written by the indexing model from the issue text.