maint: re-anchor all .gitignore entries.

* bootstrap: Copy from gnulib's latest for the fix to
automatically anchor entries it constructs.
* gnulib: Update to latest just so it has the same bootstrap.
* .gitignore, build-aux/.gitignore, doc/.gitignore:
* lib/.gitignore, m4/.gitignore, po/.gitignore:
* runtime-po/.gitignore: Re-anchor all entries.
(cherry picked from commit e503b9cbbe)

Conflicts:

    build-aux/.gitignore
    doc/.gitignore
    src/.gitignore

build-aux/.gitignore

@@ -1,24 +1,24 @@
-Makefile
-Makefile.in
-announce-gen
-arg-nonnull.h
-c++defs.h
-compile
-config.guess
-config.rpath
-config.sub
-depcomp
-gendocs.sh
-git-version-gen
-install-sh
-javacomp.sh.in
-javaexec.sh.in
-link-warning.h
-mdate-sh
-missing
-texinfo.tex
-update-copyright
-useless-if-before-free
-vc-list-files
-warn-on-use.h
-ylwrap
+/Makefile
+/Makefile.in
+/announce-gen
+/arg-nonnull.h
+/c++defs.h
+/compile
+/config.guess
+/config.rpath
+/config.sub
+/depcomp
+/gendocs.sh
+/git-version-gen
+/install-sh
+/javacomp.sh.in
+/javaexec.sh.in
+/link-warning.h
+/mdate-sh
+/missing
+/texinfo.tex
+/update-copyright
+/useless-if-before-free
+/vc-list-files
+/warn-on-use.h
+/ylwrap