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/kexecdh.c b/kexecdh.c
index c52c5e2..3115d13 100644
--- a/kexecdh.c
+++ b/kexecdh.c
@@ -26,7 +26,7 @@
 
 #include "includes.h"
 
-#ifdef OPENSSL_HAS_ECC
+#if defined(WITH_OPENSSL) && defined(OPENSSL_HAS_ECC)
 
 #include <sys/types.h>
 
@@ -94,4 +94,4 @@
 	*hash = digest;
 	*hashlen = ssh_digest_bytes(hash_alg);
 }
-#endif /* OPENSSL_HAS_ECC */
+#endif /* defined(WITH_OPENSSL) && defined(OPENSSL_HAS_ECC) */