The next release of Ubuntu will use eglibc 2.16, which drops the definition of gets (following ISO C11).
The embedded gnulib assumes that gets is defined, and thus causes a build failure in the current development release of Ubuntu.
Upstream gnulib has followed this change in http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=66712c23388e93e5c518ebc8515140fa0c807348
Workaround is to disable the warning in gl/stdio.in.h.
Please update gnulib to allow nagios-plugins to compile in environments where gets is not defined.
Ubuntu bug: https://launchpad.net/bugs/1097848