Make it possible to implement the atomics in config.h

For most cases, the GCC/Intel atomics (__sync functions) are fine,
but there are some systems for which libgcc is incorrectly built
and thus configure reports HAVE_INTEL_ATOMIC_PRIMITIVES
even though the Intel atomics aren't really implemented (i.e. QNX 6.5).

As of now, it is possible to implement the atomics in config.h
and define HB_NO_ATOMIC_IMPL to force HB use *that* implementation.

Author: KonstantinRitt

src/hb-atomic-private.hh

@@ -42,6 +42,11 @@
 #if 0
 
 
+#elif !defined(HB_NO_MT) && defined(HB_NO_ATOMIC_IMPL)
+
+/* Defined externally, i.e. in config.h */
+
+
 #elif !defined(HB_NO_MT) && (defined(_WIN32) || defined(__CYGWIN__))
 
 #include <windows.h>