[AVRO-3944] small CMake fixes (#2757)

* Bump cmake_minimum_required to 3.5.

Current version triggers a warning during configuration phase:

  CMake Deprecation Warning at CMakeLists.txt:19 (cmake_minimum_required):
    Compatibility with CMake < 3.5 will be removed from a future version of
    CMake.

    Update the VERSION argument <min> value or use a ...<max> suffix to tell
    CMake that the project does not need compatibility with older versions.

CMake 3.5.0 has been released in 2016, so it's safe to assume it's
available everywhere, which correlates with the information from
pkgs.org [1].

[1]: https://pkgs.org/search/?q=cmake&on=name

* CMake: fix the setting CMP0042 policy

The if-statement was always true since minimum cmake version was set to
3.1.
Instead, replace this condition with `if (APPLE)` since this policy
makes sense only on macOS.

lang/c++/CMakeLists.txt

@@ -16,7 +16,7 @@
 # specific language governing permissions and limitations
 # under the License.
 #
-cmake_minimum_required (VERSION 3.1)
+cmake_minimum_required (VERSION 3.5)
 
 set (CMAKE_LEGACY_CYGWIN_WIN32 0)
 
@@ -26,7 +26,7 @@ endif()
 
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
-if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.0)
+if (APPLE)
     # Enable MACOSX_RPATH by default
     cmake_policy (SET CMP0042 NEW)
 endif()