Bugzilla bug 112987: correctly detect the FreeBSD versions in which poll()

was added to libc and libc_r.  Thanks to Jeremy Lea for pointing me at
the FreeBSD Porter's Handbook.

nsprpub/pr/include/md/_freebsd.h

@@ -37,7 +37,9 @@
 
 #include "prthread.h"
 
-#include <sys/param.h>
+#if __FreeBSD__ >= 2
+#include <osreldate.h>  /* for __FreeBSD_version */
+#endif
 #include <sys/syscall.h>
 
 #define PR_LINKER_ARCH	"freebsd"
@@ -66,16 +68,23 @@
 #define _PR_HAVE_SOCKADDR_LEN
 #define _PR_STAT_HAS_ST_ATIMESPEC
 #define _PR_NO_LARGE_FILES
-#if ( __FreeBSD_version >= 220000 ) && ( __FreeBSD_version < 400008 )
-#if !defined(_PR_PTHREADS)
+
+#if defined(_PR_PTHREADS)
+#if __FreeBSD_version >= 400008
 /*
- * libc_r doesn't have poll().  Although libc has poll(), it is not
- * thread-safe so we can't use it in the pthreads version.
+ * libc_r before this version of FreeBSD doesn't have poll().
+ * Although libc has poll(), it is not thread-safe so we can't
+ * use it in the pthreads version.
  */
 #define _PR_POLL_AVAILABLE
+#endif
+#else
+#if __FreeBSD_version >= 300000
+#define _PR_POLL_AVAILABLE
 #define _PR_USE_POLL
 #endif
 #endif
+
 #define _PR_HAVE_SYSV_SEMAPHORES
 #define PR_HAVE_SYSV_NAMED_SHARED_MEMORY