completion-pcm-complete-word-inserts-delimiters is a variable defined in `minibuffer.el'.
Its value is

Treat the SPC or - inserted by `minibuffer-complete-word' as delimiters.
Those chars are treated as delimiters if this variable is non-nil.
I.e. if non-nil, M-x SPC will just insert a "-" in the minibuffer, whereas
if nil, it will list all possible commands in *Completions* because none of
the commands start with a "-" or a SPC.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.