[1.3.4] Could you detect and use the system glslang via cmake

[picca]

Hello, the idea is to build the benchmark during the Debian packaging process.

This way we should have the garantie that the provided .h files are fonctionnal.

For now I need to patch the cmake file. It would be great if out of the box cmake could detect the system gslang and use it.

[DTolm]

Hello,

I have added a check for the system glslang, hope it works for your use case.

Best regards,
Dmitrii