Remove unnecessary parsing during makefile read

* src/read.c (make_word_type): Remove w_varassign from the enum.
(get_next_mword): This function is never called on a string in a place
where we might find a variable assignment.  Any variable assignments
have already been handled via try_variable_definition(), so we don't
need to check for them here.
* tests/scripts/variables/flavors: Check triple-colon assignment in
target-specific variables.