SHADERC_SKIP_INSTALL cmake option broken

[johnzupin]

Running cmake with the SHADERC_SKIP_INSTALL option will break cmake:

...
**CMake Error: install(EXPORT "glslang-targets" ...) includes target "SPIRV" which requires target "SPIRV-Tools-opt" that is not in any export set.**
-- Generating done
CMake Generate step failed.  Build files cannot be regenerated correctly.

I think the issue is https://github.com/google/shaderc/blob/main/third_party/CMakeLists.txt#L90:

    set(GLSLANG_ENABLE_INSTALL $<NOT:${SKIP_GLSLANG_INSTALL}>)

I don't believe that "$<NOT" generator expression works during cmake's configure time, only at cmake's generation time.

[wk1093]

#1492