[LLVM] Check if zstd static libraries are built with -fPIC flag

[uditagarwal97]

Fixes: #15935

Problem
DPC++ shared lib built fails when linking static zstd libraries that are not position-independent. See #15935 for more info.

Proposed Solution
Since zstd is optional to DPC++, we check if zstd static libraries are built with -fPIC flag or not. If not, we disable zstd.

[uditagarwal97]

Once approved, I'll create a PR to upstream these changes.

[sarnex]

i havent seen this kind of issue before, is that because almost everything is built with -fPIC so it just works?

are there any other examples in llvm or other open source projects of solving it this way?

[uditagarwal97]

i havent seen this kind of issue before, is that because almost everything is built with -fPIC so it just works?

This issue was observed when DPC++ is built in shared lib configuration on Ubuntu 24.04. Apparently, the zstd static library that comes with Ubuntu 24.04 was built without the -fPIC flag and thus it gives error when linking to dynamic libraries. I've filed a bug report about the zstd package in Ubuntu 24.04 here: https://bugs.launchpad.net/ubuntu/+source/libzstd/+bug/2086543

We did not observe this error before because the zstd package that we use in CI (Ubuntu 22.04) was built correctly with fPIC flag.

are there any other examples in llvm or other open source projects of solving it this way?

I'm not aware of any. Let me dig around.

[sarnex]

yeah my overalll question is i've never seen this kind of issue before so i don't know what the fix should look like so i can't give a good review :)

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