gh-85332: Support cross-compiling for GNU/Hurd (#146094)

Recognise *-gnu (after *-linux*) as GNU/Hurd for cross-compilation.

configure

diff --git a/configure b/configure
index c5109f6..0c65e5c 100755
--- a/configure
+++ b/configure
@@ -4149,6 +4149,9 @@
 	*-*-darwin*)
 		ac_sys_system=Darwin
 		;;
+	*-gnu)
+		ac_sys_system=GNU
+		;;
 	*-*-vxworks*)
 	    ac_sys_system=VxWorks
 	    ;;
@@ -4619,6 +4622,9 @@
 			_host_ident=$host_cpu
 		esac
 		;;
+	*-gnu)
+		_host_ident=$host_cpu
+		;;
 	*-*-cygwin*)
 		_host_ident=
 		;;

configure.ac

diff --git a/configure.ac b/configure.ac
index 8fae8b4..bef76c9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -342,6 +342,9 @@
 	*-*-darwin*)
 		ac_sys_system=Darwin
 		;;
+	*-gnu)
+		ac_sys_system=GNU
+		;;
 	*-*-vxworks*)
 	    ac_sys_system=VxWorks
 	    ;;
@@ -780,6 +783,9 @@
 			_host_ident=$host_cpu
 		esac
 		;;
+	*-gnu)
+		_host_ident=$host_cpu
+		;;
 	*-*-cygwin*)
 		_host_ident=
 		;;