title: GTEST_INCLUDE_DIRS not set after find_package(GTest REQUIRED)
body: <p>I am running CMake 3.20.2 on Mac Catalina.  I have built gtest and installed it on the system with sudo make install. gtest exists in /usr/include. But when I run cmake lines:
  find_package(GTest REQUIRED)
  GTEST_FOUND is true but GTEST_INCLUDE_DIRS is not set. Under what circumstances doe GTEST_INCLUDE_DIRS either get set or not by the call to find_package?
  The <a href="https://cmake.org/cmake/help/latest/module/FindGTest.html" rel="nofollow noreferrer">CMake docs</a> say these should be set as result variables but only one seems to be set.</p>