Log Message:
Remove USE_GNU_GETTEXT from pkgsrc.  Packages generally only needed
it for plurals support, but that is already handled correctly (FSVO
"correctly") by the pkgsrc/mk/tools/msgfmt.sh script.

Also remove _USE_GNU_GETTEXT definitions from pkgsrc/mk/platform/*.mk
files as that value has been unused by pkgsrc for quite some time
(going back several branches).