Merge from gnulib (Bug#26398)

Merge from gnulib (Bug#26398)

This incorporates:
2017-04-08 getopt: prefer - to _ in new file names
2017-04-08 getopt: port recent getopt changes to macOS
* .gitignore: Add lib/getopt-cdefs.h.
* lib/getopt-cdefs.in.h: Rename from lib/getopt_cdefs.in.h.
* lib/getopt-core.h: Rename from lib/getopt_core.h.
* lib/getopt-ext.h: Rename from lib/getopt_ext.h.
* lib/getopt-pfx-core.h: Rename from lib/getopt_pfx_core.h.
* lib/getopt-pfx-ext.h: Rename from lib/getopt_pfx_ext.h.
* lib/getopt.in.h, lib/unistd.in.h, m4/getopt.m4:
Copy from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.