Fix usage with Protobuf static libraries on Unix
Hi, to be able to build an application using Protobuf as static library under Linux, I needed to fix the FindProtobuf.cmake. I was inpired by FindBoost.cmake to find correctly the static libs. I also add the Protobuf_USE_STATIC_LIBS input variable, that must be set to ON to search for static libs.
I hope these changes will be usefull for other CMake users.
Regards, S. Gallou