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.

include/json/forwards.h

@@ -25,7 +25,7 @@ class Reader;
 class CharReader;
 class CharReaderBuilder;
 
-// features.h
+// json_features.h
 class Features;
 
 // value.h

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"

include/json/reader.h

@@ -7,7 +7,7 @@
 #define CPPTL_JSON_READER_H_INCLUDED
 
 #if !defined(JSON_IS_AMALGAMATION)
-#include "features.h"
+#include "json_features.h"
 #include "value.h"
 #endif // if !defined(JSON_IS_AMALGAMATION)
 #include <deque>