Apparently, message-default-charset is deprecated, which causes the
following warning messages during the build:
In notmuch-maildir-setup-message-for-saving:
emacs/notmuch-maildir-fcc.el:172:31:Warning: ‘message-default-charset’ is an
obsolete variable (as of 26.1); The default charset comes from the
language environment
In discussion with emacs upstream over on
https://debbugs.gnu.org/35370, it appears that we can just drop this
entirely and things should still work with emacs 25.