Add ability to detect GNU/Hurd when configuring.

Adapted from https://git.hadrons.org/cgit/debian/pkgs/pciutils.git/tree/debian/patches/00-configure-hurd.patch

From e39a3af22501234a91cf28e8c57b45f9379f9101 Mon Sep 17 00:00:00 2001
From: Damien Zammit <damien@zamaudio.com>
Date: Fri, 26 Oct 2018 09:24:04 -0400
Subject: [PATCH 2/2] Add ability to detect GNU/Hurd when configuring

---
 lib/configure |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/lib/configure
+++ b/lib/configure
@@ -25,7 +25,7 @@ if [ -z "$HOST" ] ; then
 		proc=`/usr/sbin/lsdev -C -c processor -S available -F name | head -1`
 		cpu=`/usr/sbin/lsattr -F value -l $proc -a type | sed 's/_.*//'`
 	else
-		cpu=`uname -m | sed 's/^i.86$/i386/;s/^sun4u$/sparc64/;s/^i86pc$/i386/;s/^BePC$/i386/;s/^BeMac$/powerpc/;s/^BeBox$/powerpc/'`
+		cpu=`uname -m | sed 's/^i.86-AT386/i386/;s/^i.86$/i386/;s/^sun4u$/sparc64/;s/^i86pc$/i386/;s/^BePC$/i386/;s/^BeMac$/powerpc/;s/^BeBox$/powerpc/'`
 	fi
 	if [ "$sys" = "DragonFly" ]
 	then
@@ -39,6 +39,10 @@ if [ -z "$HOST" ] ; then
 	then
 		sys=cygwin
 	fi
+	if [ "$sys" = "GNU" ]
+	then
+		sys=gnu
+	fi
 	HOST=${3:-$cpu-$sys}
 fi
 [ -n "$RELEASE" ] && rel="${RELEASE}"
g</a><a href='/guix/tree/tests/status.scm?id=0d70f095964573d540da2d3fa4b7a11b251d936a'>tree</a><a href='/guix/commit/tests/status.scm?id=0d70f095964573d540da2d3fa4b7a11b251d936a'>commit</a><a href='/guix/diff/tests/status.scm?id=0d70f095964573d540da2d3fa4b7a11b251d936a'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/status.scm'>
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<div class='path'>path: <a href='/guix/log/?id=0d70f095964573d540da2d3fa4b7a11b251d936a'>root</a>/<a href='/guix/log/tests?id=0d70f095964573d540da2d3fa4b7a11b251d936a'>tests</a>/<a href='/guix/log/tests/status.scm?id=0d70f095964573d540da2d3fa4b7a11b251d936a'>status.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/status.scm?id=0d70f095964573d540da2d3fa4b7a11b251d936a&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-06-26 23:19:14 +0200'>2022-06-26</span></td><td><a href='/guix/commit/tests/status.scm?id=f99f00fc814a3e1a3e1cedb5059c896e3303677c'>status: Relay "updating substitutes" messages.</a><span class='msg-avail'>...<span class='msg-tooltip'>Until now, those messages would be accumulated and displayed all at
once, when a '\n' was finally emitted by 'guix substitute'.  In the
meantime, clients would remain silent.

* guix/status.scm (bytevector-index): Change 'number' parameter to
'numbers' and adjust accordingly.
(build-event-output-port): Pass both #\newline and #\return to
'bytevector-index'.
* tests/status.scm ("build-output-port, daemon messages with LF"): New
test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-06-26 23:19:13 +0200'>2022-06-26</span></td><td><a href='/guix/commit/tests/status.scm?id=c31605b58228dbd10c819311a17341a22c9e5118'>status: Change tests from SRFI-11 to SRFI-71.</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/status.scm: Use SRFI-71 'let' instead of SRFI-11 'let-values'.
</span></span></td><td>Ludovic Courtès</td></tr>