GOOGLETEST_VERSION is not defined in CMakeLists.txt

`GOOGLETEST_VERSION` is not defined in CMakeLists.txt. But it is used. So if I run command `mkdir mybuild && cd mybuild && cmake ..` the error happened:
```
CMake Warning at CMakeLists.txt:55 (project):
VERSION keyword not followed by a value or was followed by a value that
expanded to nothing.

CMake Error at CMakeLists.txt:130 (set_target_properties):
set_target_properties called with incorrect number of arguments.

CMake Error at CMakeLists.txt:132 (set_target_properties):
set_target_properties called with incorrect number of arguments.

-- Configuring incomplete, errors occurred!
```
So is there no `GOOGLETEST_VERSION` been defined in repo? I inserted one line `set(GOOGLETEST_VERSION 1.9.0)` and the error disappeared.