[CMake] Install library

[ProGTX]

Install the library and the headers it depends on.
Works with a shared library build (BUILD_SHARED_LIBS=ON).

In theory it works with a static library build as well,
but the installation step would have to include the SPIR-V dependencies as static libraries,
which this patch doesn't address.

Provided some installation info.

---

Example install folder:

> tree package/
package/
├── include
│   ├── libspirv2clc_export.h
│   ├── spirv
│   │   ├── 1.0
│   │   ├── 1.1
│   │   ├── 1.2
│   │   └── unified1
│   │       └── spirv.h
│   ├── spirv-tools
│   │   └── libspirv.h
│   └── spirv2clc.h
└── lib
    └── liblibspirv2clc.so

I wasn't sure how to get rid of the empty folders, but I figured there's no harm in keeping them.

Also note that this doesn't do a "proper" CMake installation, which would require exporting the targets and creating a config file, but it's the first step to making spirv2clc much easier to set up in external projects.

[kpet]

Great, thanks! The CI didn't trigger (Github has been a bit flaky these past few days) but a local build suggests everything should be fine. I'll modify the CI to also exercise installation.

[kpet]

I'm going to always enable PIC on the library. This will be required by the layer being introduced by #14. I'll tweak this after merging.