Disable libregex for QNX 7.0

`libregex` did not exist until QNX 7.1. Before that, the regex functions were part of the libc.

(apply <rust-lang#3775> to `main`)
(cherry picked from commit 0f4d2f7)

src/unix/nto/mod.rs

@@ -2864,9 +2864,9 @@ safe_f! {
 
 // Network related functions are provided by libsocket and regex
 // functions are provided by libregex.
+// Note that in QNX <=7.0, libregex functions were included it in libc itself.
 #[link(name = "socket")]
-#[link(name = "regex")]
-
+#[cfg_attr(not(target_env = "nto70"), link(name = "regex"))]
 extern "C" {
  pub fn sem_destroy(sem: *mut sem_t) -> ::c_int;
  pub fn sem_init(sem: *mut sem_t, pshared: ::c_int, value: ::c_uint) -> ::c_int;