gettext: disable tools tests when no threads

Disable the gettext-tools tests build since they break when a full
toolchain without threads is being used.
Fixes:
http://autobuild.buildroot.net/results/a187c76c359787f8e2ec6e12da2d7b1e33ed43c5/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>