Clean up crypto library abstraction in build system and source code

libssh2 used to explicitly check for libgcrypt and default to OpenSSL.

Now all possible crypto libraries are checked for explicitly, making
the addition of further crypto libraries both simpler and cleaner.

src/openssl.c

@@ -40,8 +40,6 @@
 
 #include "libssh2_priv.h"
 
-#ifndef LIBSSH2_LIBGCRYPT /* compile only if we build with OpenSSL */
-
 #include <string.h>
 
 #ifndef EVP_MAX_BLOCK_LENGTH
@@ -800,5 +798,3 @@ _libssh2_pub_priv_keyfile(LIBSSH2_SESSION *session,
     EVP_PKEY_free(pk);
     return st;
 }
-
-#endif /* !LIBSSH2_LIBGCRYPT */