Add the possibility to build without the ENGINE framework.

PR: 287

crypto/rsa/rsa.h

@@ -124,8 +124,10 @@ struct rsa_st
 	int pad;
 	long version;
 	const RSA_METHOD *meth;
+#ifndef OPENSSL_NO_ENGINE
 	/* functional reference if 'meth' is ENGINE-provided */
 	ENGINE *engine;
+#endif
 	BIGNUM *n;
 	BIGNUM *e;
 	BIGNUM *d;