Update config.guess to treat cygwin 1.7 as new system name

[Eric Blake]

The upcoming Cygwin 1.7.0 release adds so many APIs and improves so much
functionality that it probably deserves a separate designation in
config.guess.  For reference, notice the size of the most recent beta
announcement: http://cygwin.com/ml/cygwin-announce/2009-02/msg00018.html

For an example of a script in the wild that could take advantage of a
separate designation, look at line 757 in the following, which sets some
guesses when cross-compiling to cygwin:
http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=m4/printf.m4;h=4207ace;hb=f7beddb
In the example, cygwin 1.5.x lacks the printf grouping flag, but cygwin
1.7.0 provides it.

The patch deliberately avoids the extra forking of `echo | sed`, since
that is inherently slower on cygwin.  Here's the 'uname -r' output
(${UNAME_RELEASE}) between cygwin 1.5.x and cygwin 1.7.0, such that this
patch keeps i686-pc-cygwin for 1.5.x, but uses i686-pc-cygwin1.7 for 1.7.0.

1.5.25(0.156/4/2)
1.7.0(0.200/5/3)

Most decent scripts out there already use *-*-cygwin* in shell case
statements that depend on the platform name, so they will continue to
match *-*-cygwin1.7.

2009-02-26  Eric Blake  <ebb9@byu.net>

	* config.guess (*-*-cygwin1.7): Add new designation for cygwin
	1.7.0, distinct from 1.5.x.

-- 
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net
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