Fix detection of setres*id on GNU/Hurd

Like Linux, proper _SOURCE macros need to be set to get declarations of
various standard functions, notably setres*id. Now that Debian is using
-Werror=implicit-function-declaration this is really required. While at
it, define other _SOURCE macros like on GNU/Linux, since GNU/Hurd uses
the same glibc.

configure.ac

@@ -1348,6 +1348,13 @@ EOD
 	AC_DEFINE([BROKEN_SETVBUF], [1],
 	    [LynxOS has broken setvbuf() implementation])
 	;;
+*-*-gnu*)
+	dnl GNU Hurd.  Needs to be after the linux and the other *-gnu entries.
+	dnl Target SUSv3/POSIX.1-2001 plus BSD specifics.
+	dnl _DEFAULT_SOURCE is the new name for _BSD_SOURCE
+	dnl _GNU_SOURCE is needed for setres*id prototypes.
+	CPPFLAGS="$CPPFLAGS -D_XOPEN_SOURCE=600 -D_BSD_SOURCE -D_DEFAULT_SOURCE -D_GNU_SOURCE"
+	;;
 esac
 
 AC_MSG_CHECKING([compiler and flags for sanity])