Merge from origin/emacs-26

1d732d6 (origin/emacs-26) Fix gud-statement for pdb
91a68b5 ; * msdos/INSTALL: Add info about GCC versions.
7ddcc9a Document 'custom-group'
58f9e15 A minor addition to etc/DEBUG
4590414 Avoid errors in ispell.el when Enchant returns empty extra chars
d0d75f9 Make 'ispell-initialize-spellchecker-hook' work again
b90ce66 Handle selected_window change in prepare_menu_bars (Bug#31312)
79ad0b3 ; * INSTALL: Fix Emacs version number.  (Bug#31358)
91de88b Fix report-emacs-bug via mailclient on MS-Windows
f4b5ff2 Port collation tests to glibc 2.27

diff --cc etc/DEBUG
Simple merge

diff --cc lisp/net/browse-url.el
Simple merge

diff --cc lisp/progmodes/gud.el
Simple merge

diff --cc lisp/textmodes/ispell.el
Simple merge

diff --cc src/xdisp.c
Simple merge

diff --cc test/src/fns-tests.el
Simple merge