Installing libshaderc only

Hello there,

I'm building a project that depends on a recent version of libshaderc, so the version installed by the package manager on my GNU/Linux distro (Debian Testing) is not enough, so I have to build and install libshaderc manually.

Shaderc seems to be unable to use system-installed spirv-tools and glslang, so it builds them on the thirt_party directory. Oh well.

The problem is that when installing libshaderc, the local spirv-tools and glslang copies are also installed, which I don't want because I already have them on my system and I am OK with that.

This is how I configure the sources:

```
cmake .. -GNinja -DCMAKE_BUILD_TYPE=Release -DSHADERC_SKIP_TESTS=ON \
-DSPIRV_SKIP_EXECUTABLES=ON -DSKIP_GLSLANG_INSTALL=ON
```

So, is there a way to use the build system to install libshaderc ONLY? (NO spirv-tools, glslang, etc)

Thanks!