We need to append-to rather than set CXXFLAGS, otherwise we loose -O2 &
-pipe. Currently this results in zeromq being built without optimizations
at all (or whatever the compiler would default too, essentially always -O0).

Bdb is the same, for the CXX portion of its code. C code has been built
with -O2.

Boost has actually been uneffected because it receives -O3 from it's own
build flags.

Also use c++17 for protobuf

This is a backport of core#22840

gitian builds, guix build