Add features.h for feature test macros.

Because we use an amalgated build, feature
test macros should be set in a single file that
is included before any other headers, and is placed
at the top of the amalgamated build.

src/core/struct.c

@@ -21,6 +21,7 @@
 */
 
 #ifndef JANET_AMALG
+#include "features.h"
 #include <janet.h>
 #include "gc.h"
 #include "util.h"