* lisp/textmodes/flyspell.el (flyspell-case-fold-duplications): Use a
string value for the :version keyword to `defcustom'. Otherwise,
building Emacs will fail with an error like this:
Scanning ./textmodes for custom
Scanning ./url for custom
Scanning ./vc for custom
Generating ./cus-load.el...
Version must be a string
make[2]: *** [Makefile:152: cus-load.el] Error 255
This follows up to Reuben Thomas's commit 61f3a4b4fc of 10 Sep 2018.
ea9982d262