maint: cherry pick from master

[Akim Demaille]

I'm installing these changes in maint.

commit cefb538ab0523437aed094232c4b0c276aa06cfe
Author: Akim Demaille <address@hidden>
Date:   Fri Mar 6 08:25:52 2020 +0100

    gnulib: update

commit b7942f2661b625548ff1f98171ba32639d888cbd
Author: Akim Demaille <address@hidden>
Date:   Wed Mar 4 08:19:37 2020 +0100

    README: point to tests/bison, and document --trace
    
    Reported by Victor Morales Cayuela.
    
    * README, README-hacking.md: here.

commit aab3feb5a18c30c84b5f6cca3f25e54f0cb2e4a2
Author: Adrian Vogelsgesang <address@hidden>
Date:   Thu Feb 27 09:52:03 2020 +0100

    typo: succesful -> successful
    
    * data/skeletons/lalr1.cc: here
    * etc/bench.pl.in: here
    * src/location.c: and here.

commit b493c173c9cabefd2de5b7c34aac2d8c5cba4ff1
Author: Akim Demaille <address@hidden>
Date:   Wed Feb 19 19:01:14 2020 +0100

    style: remove useless declarations
    
    * src/reader.h: Don't duplicate what parse-gram.h already exposes.
    * src/lr0.h: Remove useless include.

commit b437b166037c32ed2820d14e9b15c516c5758990
Author: Akim Demaille <address@hidden>
Date:   Sun Mar 1 07:54:17 2020 +0100

    examples: use consistently the GFDL header for readmes
    
    * examples/c++/README.md, examples/c++/calc++/README.md,
    * examples/c/calc/README.md, examples/c/lexcalc/README.md,
    * examples/c/reccalc/README.md:
    Prefer the GFDL banner to the GPL one.

commit 666df338a7ee8d3472df6e15f420fd431899affa
Author: Akim Demaille <address@hidden>
Date:   Sat Feb 22 20:25:12 2020 +0100

    style: comment changes
    
    * src/symtab.h, src/lr0.c: here.

commit a4a3f08c11630a552933f00a92cce08e9c5f8c33
Author: Akim Demaille <address@hidden>
Date:   Sat Feb 15 11:16:52 2020 +0100

    doc: update recommandation for libtextstyle
    
    * README: here.

commit 192e9fdf77376a8303e4204046fd513446d840d7
Author: Akimn Demaille <address@hidden>
Date:   Sat Feb 15 10:49:14 2020 +0100

    build: fix typo
    
    * build-aux/cross-options.pl: here.