configure template-matching change

I've just checked in a rewrite of configure's method for guessing
the right template file to use when --with-template is not provided.

The old method was to look for an exact match to the $host value
(from config.guess) in template/.similar, and if that wasn't found
to strip all version information (digits and dots at the end) from
$host and try to match that against prefixes of entries in .similar.
This is clearly broken, because given .similar entries like
sparc-sun-sunos4=sunos4_gcc
sparc-sun-sunos5=solaris_sparc_gcc
it is incapable of correctly choosing the one to use for a $host
value like "sparc-sun-sunos4.1.3". After you strip the "4.1.3"
there is no basis for choosing the right .similar entry.

What I checked in tries for an exact match, and then tries to match
.similar entries to prefixes of the $host string. This is fairly
obviously the right way round, IMHO. I was able to remove some
redundant entries in .similar after making the change; for instance
i386-pc-bsdi3.0=bsdi_2.1
i386-pc-bsdi3.1=bsdi_2.1
collapse to
i386-pc-bsdi3=bsdi_2.1
which has some chance of working on bsdi 3.2 as well, if there is such
a thing.

*However*, I'm not too sure that the shell-script constructs I used
are 100% portable. If you find that configure now goes belly-up when
you leave off --with-template, please let me know.

I checked this into the 6.4 tree as well, since it is in response
to a recent complaint that 6.4 fails to configure correctly on
SunOS 4.1.3...

regards, tom lane