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

doc/.gitignore

@@ -1,28 +1,28 @@
-*.info*
-*~
-.dirstamp
-Makefile
-Makefile.in
-bison.1
-bison.aux
-bison.cp
-bison.cps
-bison.dvi
-bison.fn
-bison.html
-bison.ky
-bison.log
-bison.pdf
-bison.pg
-bison.ps
-bison.toc
-bison.tp
-bison.vr
-cross-options.texi
-gendocs_template
-refcard.dvi
-refcard.log
-refcard.ps
-stamp-vti
-version.texi
-yacc.1
+/*.info*
+/*~
+/.dirstamp
+/Makefile
+/Makefile.in
+/bison.1
+/bison.aux
+/bison.cp
+/bison.cps
+/bison.dvi
+/bison.fn
+/bison.html
+/bison.ky
+/bison.log
+/bison.pdf
+/bison.pg
+/bison.ps
+/bison.toc
+/bison.tp
+/bison.vr
+/cross-options.texi
+/gendocs_template
+/refcard.dvi
+/refcard.log
+/refcard.ps
+/stamp-vti
+/version.texi
+/yacc.1