Fix C++ compiler check to auto-disable building libasprintf
* gettext-runtime/configure.ac: To automatically disable building
libasprintf when no C++ is found, check if CXX is "no", not ":". This
seems to be a left over of commit 09963c23. Reported by Assaf Gordon in:
<https://lists.gnu.org/archive/html/bug-gettext/2014-10/msg00003.html>.
