Fix libiconv build on systems with glibc 2.16+.

Incorporate part of upstream patch (see: lib/stdio.in.h bits from
https://lists.gnu.org/archive/html/bug-gnulib/2012-03/msg00186.html)
to fix libiconv 1.14 build on systems with glibc 2.16+.  Long story
short, glibc dropped support for gets, so we need to remove the
gets declarations from srclib/stdio.in.h.