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