Detect openssl header version doesn't match runtime version

We already do this for libevent; let's do it for openssl too.

For now, I'm making it always a warn, since this has caused some
problems in the past.  Later, we can see about making it less severe.

changes/detect_openssl_mismatch

@@ -0,0 +1,4 @@
+  o Minor features:
+    - Detect when we're running with a version of OpenSSL other than the
+      one we compiled with. This has occasionally given people hard-to-
+      track-down errors.