Merge from gnulib

Merge from gnulib

This incorporates:
2017-03-14 snippets: move unadjusted snippet sources to lib
2017-03-14 gnulib-tool: fix typo in comment output
2017-03-14 snippets: work around GNU Make 3.82 VPATH
2017-03-13 gnulib-tool: minor --gnu-make fixups
2017-03-12 gnulib-tool: new option --gnu-make
* .gitignore: Remove lib/arg-nonnull.h, lib/c++defs.h,
lib/warn-on-use.h.  Change exception from
build-aux/snippet/_Noreturn.h to lib/_Noreturn.h.
* admin/authors.el (authors-renamed-files-regexps):
* admin/notes/copyright, make-dist:
The snippet files moved from build-aux/snippet to lib.
* lib/_Noreturn.h: Rename from build-aux/snippet/_Noreturn.h.
* lib/arg-nonnull.h: Rename from build-aux/snippet/arg-nonnull.h.
* lib/c++defs.h: Rename from build-aux/snippet/c++defs.h.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/warn-on-use.h: Rename from build-aux/snippet/warn-on-use.h.