support --without-openssl at configure time

Disables and removes dependency on OpenSSL. Many features don't
work and the set of crypto options is greatly restricted. This
will only work on system with native arc4random or /dev/urandom.

Considered highly experimental for now.
diff --git a/kexdhs.c b/kexdhs.c
index c3011f7..56aa5d0 100644
--- a/kexdhs.c
+++ b/kexdhs.c
@@ -25,6 +25,8 @@
 
 #include "includes.h"
 
+#ifdef WITH_OPENSSL
+
 #include <sys/types.h>
 
 #include <stdarg.h>
@@ -158,3 +160,4 @@
 	BN_clear_free(shared_secret);
 	kex_finish(kex);
 }
+#endif /* WITH_OPENSSL */