Overhaul parsing of config.guess triplet string (Issue #155) - #162

Split the string into its fields then do specific tests on each (with
numeric comparison for numeric fields) instead of doing a whole-string
string regex match each time.

The second field and the first part of the third field are constants
(and if they don't match, something is badly unsupported), the most
salient detail is the second part of the third field, and the first
field only has one or two known values (depending on the second part
of the third field). That means it is inefficient to keep restarting
the regex engine at the beginning, where it usually matches, only to
usually fail towards the end.

Instead, first do string equality tests for the known bad second and
first-part-of-third fields. Then iterate through the numeric (!string)
known second-part-of-third to quickly check this highly variable
detail. Only then do a test for first field if we get that far (thanks
to lazy evaluation of conditionals). Finally, complete the numeric
comparison for known kernel subversions.