pw_protobuf_compiler: Improve pw_proto_library in CMake
pw_protobuf_compiler: Improve pw_proto_library in CMake

Improves the pw_proto_library function in CMake to add polyfill
dependencies for pwpb libraries and to support relative
STRIP_PREFIX paths like in GN.

Change-Id: I269699b185d66085562aca520b5ef60e27471c28
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/81626
Pigweed-Auto-Submit: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed