multimedia/motion: Fix build with NLS=off: Can't exec "autopoint"

Can't exec "autopoint": No such file or directory at /usr/local/share/autoconf2.72/Autom4te/FileUtils.pm line 318.
autoreconf2.72: error: autopoint failed with exit status: 2

> Mk/Uses/autoreconf.mk: Error running /usr/local/bin/autoreconf2.72

While here remove GNU_CONFIGURE_MANPREFIX as it's a default value now.

No bump - the default build has not changed.

PR: 282531
Approved by: ports@blievers.net (maintainer)
MFH: 2025Q1