Move fallback definitions of `FLT_EPSILON` etc. after `#include <mruby/value.h>`

that includes `float.h`. It allows definitions from native headers.

include/mruby.h

@@ -70,6 +70,11 @@
 
 #include "mrbconf.h"
 
+#include <mruby/common.h>
+#include <mruby/value.h>
+#include <mruby/gc.h>
+#include <mruby/version.h>
+
 #ifndef MRB_WITHOUT_FLOAT
 #ifndef FLT_EPSILON
 #define FLT_EPSILON (1.19209290e-07f)
@@ -88,11 +93,6 @@
 #endif
 #endif
 
-#include <mruby/common.h>
-#include <mruby/value.h>
-#include <mruby/gc.h>
-#include <mruby/version.h>
-
 /**
  * MRuby C API entry point
  */