glibc 2.26 introduces some changes that make `libc-test` not work anymore. - It [removes `xlocale.h`](https://sourceware.org/glibc/wiki/Release/2.26#Removal_of_.27xlocale.h.27) - It [removes `SIGUNUSED`](http://man7.org/linux/man-pages/man7/signal.7.html)
