[MLIR] Fix building unittests in in-tree build

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D95978

mlir/CMakeLists.txt

@@ -133,7 +133,7 @@ add_subdirectory(lib)
 add_subdirectory(lib/CAPI)
 if (MLIR_INCLUDE_TESTS)
   add_definitions(-DMLIR_INCLUDE_TESTS)
-  if (TARGET gtest)
+  if (EXISTS ${LLVM_MAIN_SRC_DIR}/utils/unittest/googletest/include/gtest/gtest.h)
     add_subdirectory(unittests)
   else()
     message(WARNING "gtest not found, unittests will not be available")