Put exact version info into building instructions for OpenBSD, because gcc 4.9.3p0 throws a warning (it thinks the return value of htons is int when it actually is uint16_t).

doc/building.rst

@@ -89,7 +89,7 @@ Follow Ubuntu instructions without the ``apt-get`` commands as these are taken c
 
 Building on OpenBSD
 -------------------
-The procedure below works on OpenBSD 5.8
+The procedure below works on OpenBSD 5.8 with gcc version 4.8.4, cmake version 3.2.3 and Python version 2.7.10.
 
 - Install a recent gcc, python and cmake: