Use OpenSSL-specific ifdefs in sha2.h

In order to separate OpenSSL's SHA symbols, this header has been using
USE_SSL, which is equivalent to USE_OPENSSL.  There is now only one SSL
implementation included in the tree, so this works fine, but when
adding a new SSL implementation this would run into failures.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/0DF29010-CE26-4F51-85A6-9C8ABF5536F9@yesql.se

src/include/common/sha2.h

@@ -50,7 +50,7 @@
 #ifndef _PG_SHA2_H_
 #define _PG_SHA2_H_
 
-#ifdef USE_SSL
+#ifdef USE_OPENSSL
 #include <openssl/sha.h>
 #endif
 
@@ -69,7 +69,7 @@
 #define PG_SHA512_DIGEST_STRING_LENGTH	(PG_SHA512_DIGEST_LENGTH * 2 + 1)
 
 /* Context Structures for SHA-1/224/256/384/512 */
-#ifdef USE_SSL
+#ifdef USE_OPENSSL
 typedef SHA256_CTX pg_sha256_ctx;
 typedef SHA512_CTX pg_sha512_ctx;
 typedef SHA256_CTX pg_sha224_ctx;
@@ -89,7 +89,7 @@ typedef struct pg_sha512_ctx
 } pg_sha512_ctx;
 typedef struct pg_sha256_ctx pg_sha224_ctx;
 typedef struct pg_sha512_ctx pg_sha384_ctx;
-#endif							/* USE_SSL */
+#endif							/* USE_OPENSSL */
 
 /* Interface routines for SHA224/256/384/512 */
 extern void pg_sha224_init(pg_sha224_ctx *ctx);