Fix function body for variadic macro test.

AC_LANG_PROGRAM puts its second argument inside main() so we don't need
to do it ourselves.

configure.ac

@@ -26,7 +26,7 @@ AC_MSG_CHECKING([if $CC supports C99-style variadic macros])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 int f(int a, int b, int c) { return a + b + c; }
 #define F(a, ...) f(a, __VA_ARGS__)
-]], [[int main(void) { return F(1, 2, -3); }]])],
+]], [[return F(1, 2, -3);]])],
 	[ AC_MSG_RESULT([yes]) ],
 	[ AC_MSG_ERROR([*** OpenSSH requires support for C99-style variadic macros]) ]
 )