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

src/.gitignore

@@ -1,14 +1,14 @@
-*.flc
-*.log
-*.o
-*.output
-.deps
-.dirstamp
-Makefile
-Makefile.in
-bison
-bison.exe
-scan-code.c
-scan-gram.c
-scan-skel.c
-yacc
+/*.flc
+/*.log
+/*.o
+/*.output
+/.deps
+/.dirstamp
+/Makefile
+/Makefile.in
+/bison
+/bison.exe
+/scan-code.c
+/scan-gram.c
+/scan-skel.c
+/yacc