cmake: Do not install libgtest

[mgorny]

Summary

libgtest is an internal test dependency, and should not be installed to
the live system as a part of LLVM. Installing it top-level is just
asking for trouble, especially that LLVM has no right to claim that
specific filename and any other package may decide to install file with
the same name.

Bug: http://llvm.org/bugs/show_bug.cgi?id=18341

[vsk]

I think this excludes the gtest_main library too. Which is fine by me. LGTM.

[mgorny]

Thanks for the review. Could you commit it, please? I don't have commit access.

[vsk]

Hold on. Why is r240981 not enough to fix the problem? Is it because we still append the gtest* libraries into LLVM_EXPORTS (cmake/modules/AddLLVM.cmake +577)?

[mgorny]

Hmm, I'm going to recheck that and get back to you ASAP.

[mgorny]

I'm sorry, indeed it seems that this revision handles it, and my patch is not necessary anymore.

utils/unittest/CMakeLists.txt

if (NOT LLVM_ENABLE_THREADS)
   add_definitions( -DGTEST_HAS_PTHREAD=0 )
 endif()
 
 find_library(PTHREAD_LIBRARY_PATH pthread)
 if (PTHREAD_LIBRARY_PATH)
   list(APPEND LIBS pthread)
 endif()
 
+# Delay building until the tests pull it in, and avoid installing it
+set(EXCLUDE_FROM_ALL ON)
+
 add_llvm_library(gtest
   googletest/src/gtest-all.cc
 
   LINK_LIBS
   ${LIBS}
 
   LINK_COMPONENTS
   Support # Depends on llvm::raw_ostream
 )
 
 add_subdirectory(UnitTestMain)