Adding a warning when the GTest lib will be possibly overridden?

[Yvan Yi]

Hi there,

When building the Protocol Buffers via CMake by default options, could we print a warning to notice user the GTEst libs will be possibly overridden?

It just happened and cost much time to debug. :(

The story begins with protobuf and gTest upgrade requirement recently. After installing target version of gTest 1.15.2 and protobuf 25.3. Like other common SW installation, the options to build gTest and protobuf are default, except for enabling shared lib. (make && make install).

Some test program crashed with segmentation fault as below, (I thought it might be caused by more strict on mock object lifecycle at the begging, so spent sometime to optimize the test code...):

 
Strange issue caused by version compatibility. Have to say it again, it caused much time to debug.

(Suddenly found the path includes some keywords pointing to protobuf, then checked the version and found gTest lib was overridden by protobuf installation.)

I think It's necessary to notice user by a warning say something like GTest libs will be installed and possibly override your previously installed version. 

Please take a look on this and take actions if needed, thanks in advance.

Any discussion/suggestion is welcome.

Best regards,

Yvan