devel/c4core: New port: Library of low-level C++ utilities

c4core is a library of low-level C++ utilities, written with low-latency
projects in mind.

This is a dependency for a new port (devel/rapidyaml) which in turn is needed
for the new upcoming version of devel/jsonnet.

PR: 260996
Reported by: gasol.wu@gmail.com (maintainer)