gettext: do not hardcode system shell.

For systems like Ubuntu that use dash as default shell. Libtool
will fail when invoked with /bin/sh due to lack of += assignment.

This is acutally a upstream bug, will try to push back upstream.

Version bump gettext to lastest upstream release 0.18.2

fixes #3
modulesets: bump gettext version attribute
Merge pull request #4 from str1ngs/master

gettext: do not hardcode system shell.