autotools: bump gettext to 0.19.2 and fix Darwin compilation

Gettext does not compile on Darwin platform. The following updates
gettext to 0.19.2 (latest) version and fixes Darwin compilation
errors.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Merge pull request #906 from davidlt/fix-darwin-gettext

autotools: bump gettext to 0.19.2 and fix Darwin compilation