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/ssh-ecdsa.c b/ssh-ecdsa.c
index 1119db0..2c76f8b 100644
--- a/ssh-ecdsa.c
+++ b/ssh-ecdsa.c
@@ -26,7 +26,7 @@
 
 #include "includes.h"
 
-#ifdef OPENSSL_HAS_ECC
+#if defined(WITH_OPENSSL) && defined(OPENSSL_HAS_ECC)
 
 #include <sys/types.h>
 
@@ -189,4 +189,4 @@
 	return ret;
 }
 
-#endif /* OPENSSL_HAS_ECC */
+#endif /* WITH_OPENSSL && OPENSSL_HAS_ECC */