Remove #error from include files.

Don't have #error statements in header files, but instead wrap
the contents of that file in #ifndef OPENSSL_NO_xxx
This means it is now always safe to include the header file.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/ssl_lib.c

@@ -152,16 +152,10 @@
 #include <openssl/x509v3.h>
 #include <openssl/rand.h>
 #include <openssl/ocsp.h>
-#ifndef OPENSSL_NO_DH
-# include <openssl/dh.h>
-#endif
-#ifndef OPENSSL_NO_ENGINE
-# include <openssl/engine.h>
-#endif
+#include <openssl/dh.h>
+#include <openssl/engine.h>
 #include <openssl/async.h>
-#ifndef OPENSSL_NO_CT
-# include <openssl/ct.h>
-#endif
+#include <openssl/ct.h>
 
 const char SSL_version_str[] = OPENSSL_VERSION_TEXT;