Create presym files for each build target

Previously, presym files were always created in `build/{presym,presym.inc}`.
However, this constraint is inconvenient because it is common to use
multiple build configurations and build targets in a single mruby tree.
Therefore, change to create presym file for each build target.

src/symbol.c

@@ -20,7 +20,7 @@ static const struct {
   const char *name;
   uint16_t len;
 } presym_table[] = {
-#include <../build/presym.inc>
+#include <mruby/presym.inc>
 };
 
 static mrb_sym