Just hijacking to ask for some extra comments :-)
can we move this comment inside the if?
And mention whether we're going to link static or dynamically, or if it depends on BUILD_SHARED_LIBS
similarly, can we add a comment here? mentioning the overall "system-installed GRPC" idea,
"We always link dynamically in this mode. While the static libraries are usually installed, the CMake files telling us *which* static libraries to link are not".
nit: comma after "linking"