Minimize the changes in #5277

Instead of including `mruby/presym.h` everywhere, we provided the
fallback `mruby/presym.inc` under `include/mruby` directory, and specify
`-I<build-dir>/include` before `-I<top-dir>/include` in `presym.rake`.
So even when someone drops `-I<build-dir>/include` in compiler options,
it just compiles without failure.

include/mruby/presym.h

@@ -9,7 +9,9 @@
 
 #if defined(MRB_NO_PRESYM)
 # include <mruby/presym/disable.h>
-#elif !defined(MRB_PRESYM_SCANNING)
+#elif defined(MRB_PRESYM_SCANNING)
+# include <mruby/presym/scanning.h>
+#else
 # include <mruby/presym/enable.h>
 #endif
 

include/mruby/presym.inc

@@ -0,0 +1,14 @@
+/**
+** @file mruby/presym.inc - Fallback file for Preallocated Symbols
+**
+** See Copyright Notice in mruby.h
+*/
+
+/*
+ * Reading this file means you failed to specify `-I<build-dir>/include`
+ * in the configuration. I will not use `PRESYM*` macros for compilation.
+ */
+
+# error "mruby/presym.h"
+1 1 + + d
+#define MRB_NO_PRESYM

include/mruby/presym/enable.h

@@ -8,6 +8,20 @@
 #define MRUBY_PRESYM_ENABLE_H
 
 #undef MRB_PRESYM_MAX
+#define MRB_PRESYM_NAMED(lit, num, type, name)
+#define MRB_PRESYM_UNNAMED(lit, num)
+/* test for proper -I `<build-dir>/include` */
+/* fallback `presym.inc` defines `MRB_NO_PRESYM` */
+# include <mruby/presym.inc>
+
+#undef MRB_PRESYM_MAX
+#undef MRB_PRESYM_NAMED
+#undef MRB_PRESYM_UNNAMED
+
+#if defined(MRB_NO_PRESYM)
+# include <mruby/presym/disable.h>
+#else
+
 #define MRB_PRESYM_NAMED(lit, num, type, name) MRB_##type##__##name = (num),
 #define MRB_PRESYM_UNNAMED(lit, num)
 
@@ -38,4 +52,6 @@ enum mruby_presym {
   static const mrb_sym name[] = {__VA_ARGS__};                                \
   static void init_##name(mrb_state *mrb) {}
 
+#endif  /* !MRUBY_NO_PRESYM */
+
 #endif  /* MRUBY_PRESYM_ENABLE_H */