[Polly] Reuse LLVM's build rules for gtest/gmock

[mgorny]

Summary

Reuse LLVM's CMakeLists.txt for gtest/gmock instead of reinventing
them in Polly. This fixes a lot of linking errors due to not linking
LLVMSupport in for me.

[Meinersbur]

LGTM. Thanks.

(I think this code was once copied from clang; clang's CMakeLists.txt now also reuses LLVM's gtest CMakeLists.txt)

polly/CMakeLists.txt

execute_process(COMMAND "${LLVM_TOOLS_BINARY_DIR}/llvm-config" --src-root
       OUTPUT_STRIP_TRAILING_WHITESPACE)
     set(LLVM_SOURCE_ROOT ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
   endif()
 
   # Enable unit tests if available.
   set(POLLY_GTEST_AVAIL 0)
   set(UNITTEST_DIR ${LLVM_SOURCE_ROOT}/utils/unittest)
   if(EXISTS ${UNITTEST_DIR}/googletest/include/gtest/gtest.h)
-    # The build tree already exports the gtest target, which we can reuse
-    if (TARGET gtest)
-      # LLVM Doesn't export gtest's include directorys, so do that here
-      set_target_properties(gtest
-        PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
-        "${UNITTEST_DIR}/googletest/include;${UNITTEST_DIR}/googlemock/include"
-        )
-      set(POLLY_GTEST_AVAIL 1)
-    else()
-      add_library(gtest
-        ${UNITTEST_DIR}/googletest/src/gtest-all.cc
-        ${UNITTEST_DIR}/googlemock/src/gmock-all.cc
-        )
-      target_include_directories(gtest
-        PUBLIC
-        "${UNITTEST_DIR}/googletest/include"
-        "${UNITTEST_DIR}/googlemock/include"
-
-        PRIVATE
-        "${UNITTEST_DIR}/googletest"
-        "${UNITTEST_DIR}/googlemock"
-        )
-      target_link_libraries(gtest PUBLIC -lpthread)
-
-      add_library(gtest_main ${UNITTEST_DIR}/UnitTestMain/TestMain.cpp)
-      target_link_libraries(gtest_main PUBLIC gtest)
-
-      set(POLLY_GTEST_AVAIL 1)
-    endif()
+    if (NOT TARGET gtest)
+      add_subdirectory(${UNITTEST_DIR} utils/unittest)
+    endif()
+
   endif()
 
   # Make sure the isl c files are built as fPIC
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")
 
   # Set directory for polly-isl-test.
   set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin")
 else ()