build: add support for OpenBSD to depends

depends/patches/boost/fix_openbsd_test_lib.patch

@@ -0,0 +1,38 @@
+commit 684f067dde3b798877655cdda4eab8d7c26b2510
+Author: George Koehler <kernigh@gmail.com>
+Date:   Thu Oct 3 20:06:38 2019 -0400
+
+    OpenBSD is missing SI_ASYNCIO and SI_MESGQ
+    
+    Check if SI_ASYNCIO and SI_MESGQ are defined as macros.  This allows
+    to run tests on OpenBSD 6.5, where the macros are missing.
+    
+    This is identical to patch-boost_test_impl_execution_monitor_ipp in
+    OpenBSD Ports, except that I added a comment.
+
+    Can be removed when we use Boost 1.72.x or later.
+
+diff --git a/boost/test/impl/execution_monitor.ipp b/boost/test/impl/execution_monitor.ipp
+index ccc44972..77a01e21 100644
+--- a/boost/test/impl/execution_monitor.ipp
++++ b/boost/test/impl/execution_monitor.ipp
+@@ -391,14 +391,19 @@ system_signal_exception::report() const
+         report_error( execution_exception::system_error,
+                       "signal: the expiration of a timer set by timer_settimer()" );
+         break;
++// OpenBSD was missing SI_ASYNCIO and SI_MESGQ
++#ifdef SI_ASYNCIO
+     case SI_ASYNCIO:
+         report_error( execution_exception::system_error,
+                       "signal: generated by the completion of an asynchronous I/O request" );
+         break;
++#endif
++#ifdef SI_MESGQ
+     case SI_MESGQ:
+         report_error( execution_exception::system_error,
+                       "signal: generated by the the arrival of a message on an empty message queue" );
+         break;
++#endif
+     default:
+         break;
+     }