Fix detecting libgettextlib on Fedora 19 and Debian Jessie

Fedora, Debian, and probably other distributions do not provide an unversioned libgettextlib.so file given upstream considers it as a private library. Work around corresponding to issue #36.
Merge pull request #48 from nalimilan/patch-1

Fix detecting libgettextlib on Fedora 19 and Debian Jessie