Issue #970: Rename features.h to json_features.h (#1024)

This patch fixes a build issue on CMake, presumably due to the new glibc
having a features.h include file. This patch renames our features.h file
to avoid a name collision.
diff --git a/include/json/json.h b/include/json/json.h
index 19f14c2..f1b679a 100644
--- a/include/json/json.h
+++ b/include/json/json.h
@@ -7,7 +7,7 @@
 #define JSON_JSON_H_INCLUDED
 
 #include "autolink.h"
-#include "features.h"
+#include "json_features.h"
 #include "reader.h"
 #include "value.h"
 #include "writer.h"