Merge remote-tracking branch 'upstream/master' into aldabra

diff --cc Makefile.doc
Simple merge

diff --cc Recipe
Simple merge

diff --cc devel.but
Simple merge

diff --cc gtk.c
Simple merge

diff --cc pearl.c
Simple merge

diff --cc puzzles.but
Simple merge

diff --cc slant.c
Simple merge

diff --cc towers.c
Simple merge