Add support for find_packge?

[HolyMiss]

S2_USE_GLOG is invalid after install

[jmr]

What do you mean by "invalid"? It looks like you just need to keep building with -DS2_USE_GLOG if you want to use glog.

How should we use find_package to make this better?

[HolyMiss]

I add S2_USE_GLOG when compiling. After I make install, if I do not add S2_USE_GLOG in my code, it will report an error. If I add this macro, it will be injected into my code, so a better way is to provide find_package Mode, Add the S2_USE_GLOG macro in S2Config.cmake if it is used during compilation