Rely on jack_native_thread_t definition from systemdeps.h

types.h:
Remove the definition of `jack_native_thread_t` as we will be relying on
the one from systemdeps.h.

{jack,thread}.h:
Include systemdeps.h for definition of `jack_native_thread_t`.

jack.h

@@ -25,6 +25,7 @@
 extern "C" {
 #endif
 
+#include <jack/systemdeps.h>
 #include <jack/types.h>
 #include <jack/transport.h>
 

thread.h

@@ -25,6 +25,7 @@ extern "C" {
 #endif
 
 #include <pthread.h>
+#include <jack/systemdeps.h>
 #include <jack/weakmacros.h>
 #include <jack/types.h>
 

types.h

@@ -96,13 +96,6 @@ typedef struct _jack_client  jack_client_t;
  */
 typedef uint32_t	     jack_port_id_t;
 
-/**
- *  to make jack API independent of different thread implementations,
- *  we define jack_native_thread_t to pthread_t here.
- *  (all platforms that jack1 runs on, have pthread)
- */
-typedef pthread_t            jack_native_thread_t;
-
 /**
  *  @ref jack_options_t bits
  */