Fix pg_control handling for PostgreSQL 12

"FullTransactionId" is not present in earlier versions, so we'll
have to #ifdef that out for those.

We should probably add more elegant checks to ensure that repmgr
is being executed against the PostgreSQL version it was built against.
In a normal production environment that will be the case as the
matching package will have been installed, so version mismatches
don't usually occur.

Note that while currently most aspects of the repmgr client are
compatible across PostgreSQL versions, the repmgrd code is very
version specific, so version specificity is a given anyway.

controldata.c

@@ -312,6 +312,7 @@ get_controlfile(const char *DataDir)
 
 	if (version_num >= 120000)
 	{
+#if PG_ACTUAL_VERSION_NUM >= 120000
 		ControlFileData12 *ptr = (struct ControlFileData12 *)ControlFileDataPtr;
 		control_file_info->system_identifier = ptr->system_identifier;
 		control_file_info->state = ptr->state;
@@ -320,6 +321,10 @@ get_controlfile(const char *DataDir)
 		control_file_info->timeline = ptr->checkPointCopy.ThisTimeLineID;
 		control_file_info->minRecoveryPointTLI = ptr->minRecoveryPointTLI;
 		control_file_info->minRecoveryPoint = ptr->minRecoveryPoint;
+#else
+		fprintf(stderr, "ERROR: please use a repmgr version built for PostgreSQL 12\n");
+		exit(ERR_BAD_CONFIG);
+#endif
 	}
 	else if (version_num >= 110000)
 	{