Make OpenSSL usage thread-safe

OpenSSL can randomly segfault unless we register a callback function
to do locking.

https://www.openssl.org/docs/manmaster/crypto/threads.html

src/libmain/shared.hh

@@ -101,4 +101,8 @@ struct PrintFreed
 };
 
 
+/* Install a SIGSEGV handler to detect stack overflows. */
+void detectStackOverflow();
+
+
 }