cmake: Use stripped libraries when building packages

Previously, unstripped binaries were included in the debian packages (and others?), which resulted in long build times. #3202 solves this problem for executables, including the stripped binaries unless otherwise specified. It does not do so for the client library, however.

The reason for this omission is that we require the target to be exported, so we can't straightforwardly change the installation line to use the stripped file path instead, as that would remove the export. An alternative approach would be to build and install a stripped version of libfdb_c by default, but I'm not yet sure if there would be any objection to replacing the currently generated libraries containing debug symbols with stripped versions.

I've written up some changes that produces stripped versions in `lib` that get installed and a stripped/unstripped pair in the packages directory (this uses the old model from the make build system, generating `libfdb_c.so` and `libfdb_c.so-debug`). It's possible we could go this route, though if we do, then we should consider whether we want to do the same for all binaries.