build: update to latest gnulib

I noticed that with 0.9.4, gnulib ended up replacing pthread_sigmask
on glibc, even though glibc's works perfectly fine.  It turns out
to have been an upstream gnulib bug.

* .gnulib: Update to latest, for pthread_sigmask fix.

.gnulib @ 7f494c7d

-Subproject commit 41a7841a82351f9df3e13b6199a134004d694131
+Subproject commit 7f494c7d725db4a7f3abdef09d4070725487da67