x86_64-unknown-netbsd can be a supported platform

The top level configure.ac should include x86_64-unknown-netbsd as a supported platform. The case branch can be identical to x86_64-unknown-freebsd, etc., mutatis mutando:

--- configure.ac.dist   2008-06-03 10:21:17.000000000 -0700
+++ configure.ac        2008-06-04 18:28:37.000000000 -0700
@@ -161,6 +161,15 @@
         HostVendor_CPP='unknown'
         HostOS_CPP='openbsd'
         ;;
+amd64-*-netbsd*|x86_64-*-netbsd*)
+        HostPlatform=x86_64-unknown-netbsd
+        TargetPlatform=x86_64-unknown-netbsd
+        BuildPlatform=x86_64-unknown-netbsd
+        HostPlatform_CPP='x86_64_unknown_netbsd'
+        HostArch_CPP='x86_64'
+        HostVendor_CPP='unknown'
+        HostOS_CPP='netbsd'
+        ;;
 amd64-*-freebsd*|x86_64-*-freebsd*)
         HostPlatform=x86_64-unknown-freebsd
         TargetPlatform=x86_64-unknown-freebsd