llvm_gtest: Only install once

[tstellar]

llvm_gtest and its dependencies were being installed twice (and into two different locations depending on the cmake options).

[dtzSiFive]

Hmm, what installs the libraries after this change?

I do see them installed twice presently, this isn't latest but build log I had on hand:

-- Installing: /nix/store/2fpxb3k6n60bn4w1rqrn6dny1h3pi9ir-llvm-20240508_b52160d-lib/lib/libllvm_gtest.a
-- Installing: /nix/store/2fpxb3k6n60bn4w1rqrn6dny1h3pi9ir-llvm-20240508_b52160d-lib/lib/libllvm_gtest_main.a
-- Up-to-date: /nix/store/2fpxb3k6n60bn4w1rqrn6dny1h3pi9ir-llvm-20240508_b52160d-lib/lib/libllvm_gtest.a
-- Up-to-date: /nix/store/2fpxb3k6n60bn4w1rqrn6dny1h3pi9ir-llvm-20240508_b52160d-lib/lib/libllvm_gtest_main.a

Is that what this addresses? Thanks!

[tstellar]

Hmm, what installs the libraries after this change?

The add_llvm_library calls are generating the install targets.

Is that what this addresses? Thanks!

Yes, and there was also a bug where if you configure with -DCMAKE_INSTALL_PREFIX and without -DLLVM_LIBDIR_SUFFIX then you end up with the libraries installed into two locations, because add_llvm_library installs to lib${LLVM_LIBDIR_SUFFIX} and the custom insall targets removed in this patch install to CMAKE_INSTALL_LIBDIR which is different in this scenario.

[dtzSiFive]

Thank you for the explanation, this makes sense to me and thanks for fixing this!