Fix compiliation if glog >= 0.7

[jmccl]

Fix #2171. As I understand it 'GLOG_USE_GLOG_EXPORT' figures out the right thing to do (if you're not building glog itself).

(Tested on Manjaro.)

[nh2]

@jmccl What about the Arch patch I linked in https://github.com/facebook/folly/issues/2171#issuecomment-2185016596 ?

It doesn't set GLOG_USE_GLOG_EXPORT directly, maybe the find_package(Glog CONFIG REQUIRED) figures that out automatically?

If yes, might that be better?

[jmccl]

@nh2 Perhaps; I don't really know. I haven't looked at this in a while and the glog documentation when I looked at it wasn't particularly helpful. I've got a patch on my local stuff and I assume at some point debian will upgrade glog and FB will have to address it however they wish to do so.

[01e9]

I was able to build it after commenting this line - so need an option() to disable that line (disable tests & examples)