textproc/gnugrep: add missing dependency, build with --program-prefix=g
- Add run-time dependency on `textproc/groff': the manual page won't be rendered if groff(1) is not installed;
- By popular demand, configure the port with `--program-prefix=g' as we do for other GNU utilities. This places the binaries within the same namespace more consistently; several other non-GNU/Linux systems, e.g. NetBSD, OpenBSD, Solaris, macOS (Homebrew) are doing the same. Avoid POLA violation by creating traditionally named compatibility symlinks.
PR: 263275, 265729