Make sure that H5TS_thread_id() is available as either a function or a macro in

all configurations.

Previously it was neither declared nor defined in --disable-threadsafety
builds.  The compiler's warning got lost in the noise---I first saw the issue
because my -Werror branch stopped compiling cold---and the tests still linked
and ran.

src/H5TS.c

@@ -639,13 +639,4 @@ H5TS_create_thread(void *(*func)(void *), H5TS_attr_t *attr, void *udata)
 
 } /* H5TS_create_thread */
 
-#else  /* H5_HAVE_THREADSAFE */
-
-uint64_t
-H5TS_thread_id(void)
-{
-    return 0;
-}
-
 #endif  /* H5_HAVE_THREADSAFE */
-

src/H5private.h

@@ -1915,12 +1915,14 @@ H5_DLL double H5_trace(const double *calltime, const char *func, const char *typ
 /* global library version information string */
 extern char  H5_lib_vers_info_g[];
 
+/* Include required thread-safety header, always, for the H5TS_thread_id()
+ * definition.
+ */
+#include "H5TSprivate.h"
+
 /* Lock headers */
 #ifdef H5_HAVE_THREADSAFE
 
-/* Include required thread-safety header */
-#include "H5TSprivate.h"
-
 /* replacement structure for original global variable */
 typedef struct H5_api_struct {
     H5TS_mutex_t init_lock;  /* API entrance mutex */