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/util.c

@@ -20,15 +20,16 @@
 * IN THE SOFTWARE.
 */
 
-#include <inttypes.h>
-
 #ifndef JANET_AMALG
+#include "features.h"
 #include <janet.h>
 #include "util.h"
 #include "state.h"
 #include "gc.h"
 #endif
 
+#include <inttypes.h>
+
 /* Base 64 lookup table for digits */
 const char janet_base64[65] =
     "0123456789"