Add CMake option BUILD_TESTS (#333)

Add an option BUILD_TESTS to decide whether to build S2 unit tests.

Default is with tests.

CMakeLists.txt

@@ -32,6 +32,7 @@ add_feature_info(SHARED_LIBS BUILD_SHARED_LIBS
                  "builds shared libraries instead of static.")
 
 option(BUILD_EXAMPLES "Build s2 documentation examples." ON)
+option(BUILD_TESTS "Build s2 unittests." ON)
 
 option(WITH_PYTHON "Add python interface" OFF)
 add_feature_info(PYTHON WITH_PYTHON "provides python interface to S2")
@@ -431,8 +432,12 @@ install(TARGETS ${S2_TARGETS}
         ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}"
         LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}")
 
-message("GOOGLETEST_ROOT: ${GOOGLETEST_ROOT}")
-if (GOOGLETEST_ROOT)
+if (BUILD_TESTS)
+  if (NOT GOOGLETEST_ROOT)
+    message(FATAL_ERROR "BUILD_TESTS requires GOOGLETEST_ROOT")
+  endif()
+  message("GOOGLETEST_ROOT: ${GOOGLETEST_ROOT}")
+
   add_subdirectory(${GOOGLETEST_ROOT}/googlemock build_gmock)
   include_directories(${GOOGLETEST_ROOT}/googlemock/include)
   include_directories(${GOOGLETEST_ROOT}/googletest/include)