Preserve backwards compatibility for old rust versions

As mentioned in #66, the new release will just be a patch release. To
be on the safe side, we can preserve backwards compatibility by working
around the following two incompatibilities:

* omitting the field name when initializing `Sel`, which requires rust 1.17
* using `AtomicPtr::new` in a constant, which requires rust 1.24

src/macros.rs

@@ -18,13 +18,12 @@ macro_rules! class {
         #[inline(always)]
         fn get_class(name: &str) -> Option<&'static $crate::runtime::Class> {
             unsafe {
-                static CLASS: ::std::sync::atomic::AtomicPtr<$crate::runtime::Class> =
-                    ::std::sync::atomic::AtomicPtr::new(0 as *mut _);
+                static CLASS: ::std::sync::atomic::AtomicUsize = ::std::sync::atomic::ATOMIC_USIZE_INIT;
                 // `Relaxed` should be fine since `objc_getClass` is thread-safe.
-                let ptr = CLASS.load(::std::sync::atomic::Ordering::Relaxed);
+                let ptr = CLASS.load(::std::sync::atomic::Ordering::Relaxed) as *const $crate::runtime::Class;
                 if ptr.is_null() {
                     let cls = $crate::runtime::objc_getClass(name.as_ptr() as *const _);
-                    CLASS.store(cls as *mut _, ::std::sync::atomic::Ordering::Relaxed);
+                    CLASS.store(cls as usize, ::std::sync::atomic::Ordering::Relaxed);
                     if cls.is_null() { None } else { Some(&*cls) }
                 } else {
                     Some(&*ptr)
@@ -47,14 +46,13 @@ macro_rules! sel_impl {
         #[inline(always)]
         fn register_sel(name: &str) -> $crate::runtime::Sel {
             unsafe {
-                static SEL: ::std::sync::atomic::AtomicPtr<::std::os::raw::c_void> =
-                    ::std::sync::atomic::AtomicPtr::new(0 as *mut _);
-                let ptr = SEL.load(::std::sync::atomic::Ordering::Relaxed);
+                static SEL: ::std::sync::atomic::AtomicUsize = ::std::sync::atomic::ATOMIC_USIZE_INIT;
+                let ptr = SEL.load(::std::sync::atomic::Ordering::Relaxed) as *const ::std::os::raw::c_void;
                 // It should be fine to use `Relaxed` ordering here because `sel_registerName` is
                 // thread-safe.
                 if ptr.is_null() {
                     let sel = $crate::runtime::sel_registerName(name.as_ptr() as *const _);
-                    SEL.store(sel.as_ptr() as *mut _, ::std::sync::atomic::Ordering::Relaxed);
+                    SEL.store(sel.as_ptr() as usize, ::std::sync::atomic::Ordering::Relaxed);
                     sel
                 } else {
                     $crate::runtime::Sel::from_ptr(ptr)

src/runtime.rs

@@ -154,7 +154,7 @@ impl Sel {
     #[inline]
     pub unsafe fn from_ptr(ptr: *const c_void) -> Sel {
         Sel {
-            ptr,
+            ptr: ptr,
         }
     }