build: Fix build failure on Solaris
* gettext-tools/src/cldr-plurals.c: Include "basename.h".
(main): Place optind on the right hand side of the expression, to
work around build failure on Solaris.
Reported by Dagobert Michelsen in:
<https://lists.gnu.org/archive/html/bug-gettext/2015-06/msg00015.html>.
