Inability to use system versions of dependencies.

[Zopolis4]

Currently, there is no way to use the system versions of packages, only the ones provided in externals.

This was partially addressed in #5422, but was then reverted.

Looking at the DEPS file, it looks like abseil_cpp, effcee, googletest, protobuf, re2, spirv-headers are only provided as externals.

[s-perron]

We will accept a change like #5422 if you want to pick it up, and make the suggested changes.

The priorities would have to be:

1. Pick up spir-v header from the SPIRV-Headers_SOURCE_DIR (or similar variable for other packages). If this is set do not call add_subdirectory.
2. If the directory is found in external, then use that.
3. Otherwise look for the installed package.

The Khronos spir-v project have not kept up-to-date with new version numbers. The spir-v headers version number in cmake has not been updated since Oct 11, 2021. This will cause problems, and I will not be imposing more steps on the current maintainers to support using packages.

This can cause problems, because we keep the deps that we support as close to tip-of-tree as possible, with not guarantee of working with older version, or even official releases.

All this to say, we discourage the use of install packages to build spirv-tools. If this change is made, update the README accordingly.

DEPS is used to keep track the one version of each dependency that we guarantee will work with that commit of spirv-tools. You can checkout that version using the git-sync-deps tool. If there is an error using any other mechanism, it is user error.