gettext: install ABOUT-NLS for the host

gettext: install ABOUT-NLS for the host

gettextize copies a number of files from usr/share/gettext into the project
when it is run.  One of these is ABOUT-NLS, which is simply a text file
explaining some details about gettext - So it isn't really critical but
autoreconf fails if gettext functionality is used and ABOUT-NLS is missing.

Normally this isn't an issue as the release tarball typically already
contains an ABOUT-NLS file, but as this is a generated file it normally
isn't checked into git so it fails for packages from git snapshots using
<foo>_GETTEXTIZE = YES (E.G. libuio):

configure.ac:42: error: required file './ABOUT-NLS' not found

As is evident from GETTEXT_REMOVE_UNNEEDED, this file normally gets
installed by gettext-runtime, but as we don't build/install that for the
host instead install it by hand.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>