better detection of broken -fzero-call-used-regs

gcc 13.2.0 on ppc64le refuses to compile some function, including
cipher.c:compression_alg_list() with an error:

> sorry, unimplemented: argument ‘used’ is not supportedcw
> for ‘-fzero-call-used-regs’ on this target

This extends the autoconf will-it-work test with a similarly-
structured function that seems to catch this.

Spotted/tested by Colin Watson; bz3645

Author: djmdjm

m4/openssh.m4

@@ -20,18 +20,24 @@ char *f2(char *s, ...) {
 	va_end(args);
 	return strdup(ret);
 }
+const char *f3(int s) {
+	return s ? "good" : "gooder";
+}
 int main(int argc, char **argv) {
-	(void)argv;
 	char b[256], *cp;
+	const char *s;
 	/* Some math to catch -ftrapv problems in the toolchain */
 	int i = 123 * argc, j = 456 + argc, k = 789 - argc;
 	float l = i * 2.1;
 	double m = l / 0.5;
 	long long int n = argc * 12345LL, o = 12345LL * (long long int)argc;
+	(void)argv;
 	f(1);
-	snprintf(b, sizeof b, "%d %d %d %f %f %lld %lld\n", i,j,k,l,m,n,o);
+	s = f3(f(2));
+	snprintf(b, sizeof b, "%d %d %d %f %f %lld %lld %s\n", i,j,k,l,m,n,o,s);
 	if (write(1, b, 0) == -1) exit(0);
-	cp = f2("%d %d %d %f %f %lld %lld\n", i,j,k,l,m,n,o);
+	cp = f2("%d %d %d %f %f %lld %lld %s\n", i,j,k,l,m,n,o,s);
+	if (write(1, cp, 0) == -1) exit(0);
 	free(cp);
 	/*
 	 * Test fallthrough behaviour.  clang 10's -Wimplicit-fallthrough does