Don't call pthread_condattr_setclock() unless it exists

Fixes bug 17819; bugfix on 0.2.6.3-alpha (specifically, d684dbb0).

changes/bug17819

@@ -0,0 +1,4 @@
+  o Minor bugfixes (compilation):
+    - Don't try to use the pthrad_condattr_setclock() function unless
+      it actually exists.  Fixes compilation on NetBSD-6.x. Fixes bug
+      17819; bugfix on 0.2.6.3-alpha.

configure.ac

@@ -425,6 +425,7 @@ AC_CHECK_FUNCS(
 if test "$bwin32" != true; then
   AC_CHECK_HEADERS(pthread.h)
   AC_CHECK_FUNCS(pthread_create)
+  AC_CHECK_FUNCS(pthread_condattr_setclock)
 fi
 
 if test "$bwin32" = true; then

src/common/compat_pthreads.c

@@ -185,7 +185,8 @@ tor_cond_init(tor_cond_t *cond)
     return -1;
   }
 
-#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
+#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC) \
+  && defined(HAVE_PTHREAD_CONDATTR_SETCLOCK)
   /* Use monotonic time so when we timedwait() on it, any clock adjustment
    * won't affect the timeout value. */
   if (pthread_condattr_setclock(&condattr, CLOCK_MONOTONIC)) {