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/amalgamate.py b/amalgamate.py
index 7b41e2e..fedee1e 100755
--- a/amalgamate.py
+++ b/amalgamate.py
@@ -75,7 +75,7 @@
     header.add_file(os.path.join(INCLUDE_PATH, "allocator.h"))
     header.add_file(os.path.join(INCLUDE_PATH, "config.h"))
     header.add_file(os.path.join(INCLUDE_PATH, "forwards.h"))
-    header.add_file(os.path.join(INCLUDE_PATH, "features.h"))
+    header.add_file(os.path.join(INCLUDE_PATH, "json_features.h"))
     header.add_file(os.path.join(INCLUDE_PATH, "value.h"))
     header.add_file(os.path.join(INCLUDE_PATH, "reader.h"))
     header.add_file(os.path.join(INCLUDE_PATH, "writer.h"))