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/meson.build b/meson.build
index d1f4989..65dcffa 100644
--- a/meson.build
+++ b/meson.build
@@ -23,7 +23,7 @@
   'include/json/assertions.h',
   'include/json/autolink.h',
   'include/json/config.h',
-  'include/json/features.h',
+  'include/json/json_features.h',
   'include/json/forwards.h',
   'include/json/json.h',
   'include/json/reader.h',