Print CMake warning when tag info is not available

[jkwak-work]

Problem Description

With the following recent renaming of Slang, the Slang users will get an invalid version number on the slang binary name when locally built without git-tag info fetched.

• Rename library from slang to slang-compiler and version library names #8746

Preferred Solution

We should print a warning that warns the Slang users to fetch the tag when they are building Slang locally.

Alternative Solutions

We may want to make this a CMake error if the invalid version number can cause a problem.

[jhelferty-nv]

Warnings should be visible after #9020.

This isn't currently an error because CI is often unable to see the tags. This is a result of the following from the ci.yml:

      - uses: actions/checkout@v4
        with:
          fetch-depth: "2"

Since we don't fetch much git history, I don't think git describe will be able to 'see' the most recent version tag, and thus we'd often end up falling back to v0.0.0. I was considering altering fetch-depth to be 100 or so, but that makes whether or not the version is used in CI unpredictable, so I left it alone.

(I have seen this first-hand with the CI slangpy tests for slang. There, we have to do some funky library renaming shenanigans, in order to overlay the CI-built slang libraries onto a slangpy install. So the logs there show that we rename the v0.0.0 libraries we built in CI to overwrite the slang library version that's embedded in slangpy...)

[jhelferty-nv]

I believe the changes in #9020 should address this problem, if not, please reopen.