Fix build when scm_t_timespec is different from struct timespec

Reported by Roland Haeder.  The declaration and definition of
scm_pthread_cond_timedwait were using possibly different types for the
third arg.

* THANKS: Added Roland Haeder.

* libguile/threads.h (scm_pthread_cond_timedwait): Use scm_t_timespec
  for third arg rather than struct timespec, for consistency with the
  function implementation.

NEWS

@@ -11,6 +11,7 @@ Changes in 1.8.7 (since 1.8.6)
 
 ** Fix %fast-slot-ref/set!, to avoid possible segmentation fault
 ** Fix MinGW build problem caused by HAVE_STRUCT_TIMESPEC confusion
+** Fix build problem when scm_t_timespec is different from struct timespec
 
 
 Changes in 1.8.6 (since 1.8.5)

THANKS

@@ -43,6 +43,7 @@ For fixes or providing information which led to a fix:
            John Goerzen
            Mike Gran
          Szavai Gyula
+         Roland Haeder
 	   Sven Hartrumpf
 	   Eric Hanchrow
             Sam Hocevar

libguile/threads.h

@@ -205,7 +205,7 @@ SCM_API int scm_pthread_cond_wait (pthread_cond_t *cond,
 				   pthread_mutex_t *mutex);
 SCM_API int scm_pthread_cond_timedwait (pthread_cond_t *cond,
 					pthread_mutex_t *mutex,
-					const struct timespec *abstime);
+					const scm_t_timespec *abstime);
 #endif
 
 /* More convenience functions.