Avoid Gnulib std-gnu11 module.

It has come to my attention that this module rewrites AC_PROG_CC in
a way that actually breaks Automake's AM_PROG_CC_C_O functionality.
This results in the "compile" script not being included or used in
packages bootstrapped with Autoconf 2.69.

With later versions of Autoconf things work because this module
doesn't touch things and thus disables itself.

I don't care about building with C11 one way or the other.  Let's
just skip the module.