Allow gnu_time64_abi and set it with set_cfg

Author: snogge

build.rs

@@ -13,6 +13,7 @@ const ALLOWED_CFGS: &'static [&'static str] = &[
     "freebsd12",
     "freebsd13",
     "freebsd14",
+    "gnu_time64_abi",
     "libc_align",
     "libc_cfg_target_vendor",
     "libc_const_extern_fn",
@@ -81,7 +82,7 @@ fn main() {
 
     // Some ABIs need to redirect time related symbols to their time64 equivalents.
     if is_gnu_time64_abi() {
-        println!("cargo:rustc-cfg=gnu_time64_abi");
+        set_cfg("gnu_time64_abi");
     }
 
     // On CI: deny all warnings