Skip check_numassignref_ops test for old rustc

Author: sshilovsky

build.rs

@@ -18,6 +18,7 @@ fn main() {
 
     ac.emit_expression_cfg("1u32.reverse_bits()", "has_reverse_bits");
     ac.emit_expression_cfg("1u32.trailing_ones()", "has_leading_trailing_ones");
+    ac.emit_expression_cfg("{ let mut x = 1; x += &2; }", "has_int_assignop_ref");
 
     autocfg::rerun_path("build.rs");
 }

src/lib.rs

@@ -624,6 +624,7 @@ fn check_numassign_ops() {
     assert_eq!(compute(1, 2), 1)
 }
 
+#[cfg(has_int_assignop_ref)]
 #[test]
 fn check_numassignref_ops() {
     fn compute<T: NumAssignRef + Copy>(mut x: T, y: &T) -> T {