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/moduli.c b/moduli.c
index 474caca..729d55c 100644
--- a/moduli.c
+++ b/moduli.c
@@ -39,6 +39,8 @@
 
 #include "includes.h"
 
+#ifdef WITH_OPENSSL
+
 #include <sys/param.h>
 #include <sys/types.h>
 
@@ -802,3 +804,5 @@
 
 	return (res);
 }
+
+#endif /* WITH_OPENSSL */