How to build against a given (prebuilt) version of glslang

Hi, I'm asking because I need to work against a version of `glslang` (and `spirv-tools` and `spirv-headers`) from another tree where I build Vulkan (VulkanTools, specifically, which contains a submodule of `Vulkan-LoaderAndValidationLayers`, which "imports" and builds in-tree `glslang` and its dependencies using `update_glslang_sources.sh`).
Wouldn't it be possible to build against an _installed_ version of these dependencies?
I mean, I would like to build glslang (with or without Vulkan drivers and tools) and its dependencies, and then install them to, say, `usr/local` or `/tmp/glslang-debug`, and then build `shaderc` against those built artifacts (headers, libraries, binaries).
I tried modifying `third_party/CMakeLists.txt` to use the existing versions of the 3rd-party dependencies, but I was not really successful. The CMake files of `shaderc` expect to include those dependencies as sources, rather than installed products. Has anyone done this?
Thanks