Fix cmake builds when cmake version < 3.10 (#32)

Summary:
Fix for the first part of #31

Test strategy:
Build with the fix, no longer get the error:
```
CMake Error at common/CMakeLists.txt:56 (gtest_discover_tests):
  Unknown CMake command "gtest_discover_tests".
```
Pull Request resolved: #32

Reviewed By: Zepheus

Differential Revision: D10216424

Pulled By: calebmarchent

fbshipit-source-id: 8d462a4254a9d5516ac2b00f1b062a6b7b8a7b77

logdevice/CMakeLists.txt

@@ -20,7 +20,11 @@ set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 project(${PACKAGE_NAME} CXX)
 
 enable_testing()
-include(GoogleTest OPTIONAL RESULT_VARIABLE HAVE_CMAKE_GTEST)
+# gtest_discover_tests was introduced in cmake 3.10
+if(${CMAKE_VERSION} VERSION_EQUAL "3.10" OR ${CMAKE_VERSION} VERSION_GREATER "3.10")
+  include(GoogleTest OPTIONAL RESULT_VARIABLE HAVE_CMAKE_GTEST)
+endif()
+
 include (logdevice-functions)
 
 include(build-config)