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

Use OSSH_CHECK_CFLAG_LINK() for detection of these flags and extend
test program to exercise varargs, which seems to catch more stuff.

ok dtucker@

configure.ac

@@ -234,9 +234,9 @@ if test "$GCC" = "yes" || test "$GCC" = "egcs"; then
 	# clang 17 has a different bug that causes an ICE when using this
 	# flag at all (https://bugzilla.mindrot.org/show_bug.cgi?id=3629)
 	case "$CLANG_VER" in
-	apple-15*) OSSH_CHECK_CFLAG_COMPILE([-fzero-call-used-regs=used]) ;;
+	apple-15*) OSSH_CHECK_CFLAG_LINK([-fzero-call-used-regs=used]) ;;
 	17*)	;;
-	*)	OSSH_CHECK_CFLAG_COMPILE([-fzero-call-used-regs=used]) ;;
+	*)	OSSH_CHECK_CFLAG_LINK([-fzero-call-used-regs=used]) ;;
 	esac
 	OSSH_CHECK_CFLAG_COMPILE([-ftrivial-auto-var-init=zero])
     fi

m4/openssh.m4

@@ -6,13 +6,23 @@ dnl behaviours.
 AC_DEFUN([OSSH_COMPILER_FLAG_TEST_PROGRAM],
 	[AC_LANG_SOURCE([[
 #include <stdlib.h>
+#include <stdarg.h>
 #include <stdio.h>
+#include <string.h>
 #include <unistd.h>
 /* Trivial function to help test for -fzero-call-used-regs */
 int f(int n) {return rand() % n;}
+char *f2(char *s, ...) {
+	char ret[64];
+	va_list args;
+	va_start(args, s);
+	vsnprintf(ret, sizeof(ret), s, args);
+	va_end(args);
+	return strdup(ret);
+}
 int main(int argc, char **argv) {
 	(void)argv;
-	char b[256];
+	char b[256], *cp;
 	/* Some math to catch -ftrapv problems in the toolchain */
 	int i = 123 * argc, j = 456 + argc, k = 789 - argc;
 	float l = i * 2.1;
@@ -21,6 +31,8 @@ int main(int argc, char **argv) {
 	f(1);
 	snprintf(b, sizeof b, "%d %d %d %f %f %lld %lld\n", i,j,k,l,m,n,o);
 	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);
+	free(cp);
 	/*
 	 * Test fallthrough behaviour.  clang 10's -Wimplicit-fallthrough does
 	 * not understand comments and we don't use the "fallthrough" attribute