configure: set PATH_SEPARATOR to colon for PATH w/o separator

The logic tries to figure out what the path separator in the $PATH
variable is, but if there's only one directory in the $PATH it
fails. This change make configure *guess* on colon instead of erroring
out, simply because that is probably the more common character.

PATH_SEPARATOR can always be set by the user to override the guessing.

(tricky bug to reproduce, as in my case for example the configure script
requires binaries in more than one directory so passing in a PATH with a
single dir fails.)

Reported-by: Earnestly on github
Fixes #2202
Closes #2265