x11-toolkits/libXaw: update to 1.0.16

Remove GNU_CONFIGURE_MANPREFIX

Changes: https://gitlab.freedesktop.org/xorg/lib/libxaw/-/compare/libXaw-1.0.14...libXaw-1.0.16
Reviewed by: diizzy, jbeich
Tested by: arrowd
Pull Request: https://github.com/freebsd/freebsd-ports/pull/273
Sponsored by: Serenity Cybersecurity, LLC