Add install targets for gtest

[tstellar]

Summary

Stand-alone builds need an installed version of gtest in order to run
the unittests.

[mgorny]

I'll try to test this today. I suppose the main idea would be to enable LLVM_INSTALL_GTEST, then stop unpacking third-party directory when building other packages, correct? Perhaps it would make sense to update the checks like the one in clang while doing that:

if( CLANG_INCLUDE_TESTS )
  if(EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest/googletest/include/gtest/gtest.h)
    add_subdirectory(unittests)
    ...

[mgorny]

Plus there's the matter of LLVMTestingSupport library.

[tstellar]

In D137890#3942748, @mgorny wrote:

I'll try to test this today. I suppose the main idea would be to enable LLVM_INSTALL_GTEST, then stop unpacking third-party directory when building other packages, correct? Perhaps it would make sense to update the checks like the one in clang while doing that:

if( CLANG_INCLUDE_TESTS )
  if(EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest/googletest/include/gtest/gtest.h)
    add_subdirectory(unittests)
    ...

I submitted the clang patch here: D138472

[mgorny]

third-party/unittest/CMakeLists.txt

81

Any reason you think this should be installed separately rather than included in the usual LLVM export list? I think the latter would make it more consistent to use in in-tree vs standalone builds (as you wouldn't have to find_package it).

[tstellar]

third-party/unittest/CMakeLists.txt

81

The problem with having it in the main export list is that it then it would have to be present on disk for users to be able to load LLVMConfig.cmake.

81

I was just trying to make the patch more self-contained, so it wouldn't risk breaking other build configurations, but I can add it into the main export list if you think that's better.

[mgorny]

third-party/unittest/CMakeLists.txt

81

I suppose you're thinking of packaging it separate from llvm-devel then? If not, then I think it'd be better to keep it simple and include it in main exports. After all, most users simply won't enable the option to install it, so shouldn't be affected.

[kwk]

Works for me.