GNU Make Convert Spaces to Colons

The only tricky part here is to define a literal space:

space := $(subst ,, )

SPATHS := /usr/bin/foo /usr/bin/baz /usr/bin/baz
CPATHS := $(subst $(space),:,$(SPATHS))

The shortest way to get a literal space would be via $() $(). Thus:

$(subst $() $(),:,$(CPATHS))

Or, for brevity:

_=$() $()
$(subst $(_),:,$(CPATHS))

It is perhaps an interesting curiosity that the same trick works with cmake's macros, i.e. that ${} is a separator but introduces no whitespace by itself.