build-sys:

While the previous commit checked for a GNU environment so Hurd
compilied ok, this tripped up Cygwin. configure now explicitly tests
for the structure field rather than trying to guess through compilier
flags about what the environment tells us about signals.h

References:
 commit d39d9db079

Signed-off-by: Craig Small <csmall@dropbear.xyz>

lib/test_process.c

@@ -62,9 +62,9 @@ signal_handler(int signum, siginfo_t *siginfo, void *ucontext)
 	    printf("SIG %s\n", signame);
 	    break;
 	case SI_QUEUE:
-#	    ifndef __GNU__
+#ifdef HAVE_SIGINFO_T_SI_INT
 	        printf("SIG %s value=%d\n", signame, siginfo->si_int);
-#	    else
+#else
 	        printf("case SI_QUEUE: SIG %s siginfo->si_int undefined\n", signame);
 #endif
 	    break;