Check that atomic_load works with a const _Atomic

The original C11 spec does not allow atomic_load to work on const _Atomic
variables, which we sometimes do in alGet* methods, despite the fact that it
does not modify the variable. An update to the C spec corrects this oversight,
and GCC 4.9 has allowed it anyway, while Clang 3.4 does not.

CMakeLists.txt

@@ -186,7 +186,7 @@ HAVE_C11_ALIGNAS)
 # Check if we have C11 _Atomic
 CHECK_C_SOURCE_COMPILES(
 "#include <stdatomic.h>
- int _Atomic foo;
+ const int _Atomic foo = ATOMIC_VAR_INIT(~0);
  int main()
  {
      return atomic_load(&foo);