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/math.c

@@ -20,13 +20,14 @@
 * IN THE SOFTWARE.
 */
 
-#include <math.h>
-
 #ifndef JANET_AMALG
+#include "features.h"
 #include <janet.h>
 #include "util.h"
 #endif
 
+#include <math.h>
+
 static JANET_THREAD_LOCAL JanetRNG janet_vm_rng = {0, 0, 0, 0, 0};
 
 static int janet_rng_get(void *p, Janet key, Janet *out);