decouple internal.h headers

Saves comitters' daily life by avoid #include-ing everything from
internal.h to make each file do so instead.  This would significantly
speed up incremental builds.

We take the following inclusion order in this changeset:

1.  "ruby/config.h", where _GNU_SOURCE is defined (must be the very
    first thing among everything).
2.  RUBY_EXTCONF_H if any.
3.  Standard C headers, sorted alphabetically.
4.  Other system headers, maybe guarded by #ifdef
5.  Everything else, sorted alphabetically.

Exceptions are those win32-related headers, which tend not be self-
containing (headers have inclusion order dependencies).

internal.h

@@ -18,17 +18,9 @@
 # error not for C++
 #endif
 
-#include "ruby/encoding.h"
-#include "ruby/io.h"
-#include "internal/stdbool.h"
-#include "internal/bits.h"
-
 #define LIKELY(x) RB_LIKELY(x)
 #define UNLIKELY(x) RB_UNLIKELY(x)
 
-#include "internal/compilers.h"
-#include "internal/sanitizers.h"
-
 #define numberof(array) ((int)(sizeof(array) / sizeof((array)[0])))
 #define roomof(x, y) (((x) + (y) - 1) / (y))
 #define type_roomof(x, y) roomof(sizeof(x), sizeof(y))
@@ -36,54 +28,50 @@
 /* Prevent compiler from reordering access */
 #define ACCESS_ONCE(type,x) (*((volatile type *)&(x)))
 
-#include "internal/serial.h"
-#include "internal/static_assert.h"
-#include "internal/time.h"
-#include "internal/fixnum.h"
-#include "internal/bignum.h"
-#include "internal/rational.h"
-#include "internal/numeric.h"
-#include "internal/complex.h"
-#include "internal/hash.h"
-#include "internal/missing.h"
-#include "internal/struct.h"
-#include "internal/class.h"
-#include "internal/imemo.h"
-#include "internal/compar.h"
-#include "internal/variable.h"
-#include "internal/array.h"
-#include "internal/debug.h"
-#include "internal/compile.h"
-#include "internal/cont.h"
-#include "internal/dir.h"
-#include "internal/encoding.h"
-#include "internal/enum.h"
-#include "internal/eval.h"
-#include "internal/error.h"
-#include "internal/file.h"
-#include "internal/gc.h"
-#include "internal/io.h"
-#include "internal/load.h"
-#include "internal/loadpath.h"
-#include "internal/math.h"
-#include "internal/mjit.h"
-#include "internal/object.h"
-#include "internal/parse.h"
-#include "internal/proc.h"
-#include "internal/process.h"
-#include "internal/range.h"
-#include "internal/re.h"
-#include "internal/signal.h"
-#include "internal/string.h"
-#include "internal/symbol.h"
-#include "internal/thread.h"
-#include "internal/transcode.h"
-#include "internal/enc.h"
-#include "internal/util.h"
-#include "internal/vm.h"
-#include "internal/enumerator.h"
-#include "internal/random.h"
-#include "internal/inits.h"
-#include "internal/warnings.h"
+#include "ruby/ruby.h"
+
+/* Folowing macros were formerlly defined in this header but moved to somewhere
+ * else.  In order to detect them we undef here. */
+
+/* internal/error.h */
+#undef Check_Type
+
+/* internal/class.h */
+#undef RClass
+#undef RCLASS_SUPER
+
+/* internal/gc.h */
+#undef NEWOBJ_OF
+#undef RB_NEWOBJ_OF
+#undef RB_OBJ_WRITE
+
+/* internal/hash.h */
+#undef RHASH_IFNONE
+#undef RHASH_SIZE
+
+/* internal/struct.h */
+#undef RSTRUCT_LEN
+#undef RSTRUCT_PTR
+#undef RSTRUCT_SET
+#undef RSTRUCT_GET
+
+/* Also,  we  keep  the  following  macros  here.   They  are  expected  to  be
+ * overridden in each headers. */
+
+/* internal/array.h */
+#define rb_ary_new_from_args(...) rb_nonexistent_symbol(__VA_ARGS__)
+
+/* internal/io.h */
+#define rb_io_fptr_finalize(...) rb_nonexistent_symbol(__VA_ARGS__)
+
+/* internal/string.h */
+#define rb_fstring_cstr(...) rb_nonexistent_symbol(__VA_ARGS__)
+
+/* internal/symbol.h */
+#define rb_sym_intern_ascii_cstr(...) rb_nonexistent_symbol(__VA_ARGS__)
+
+/* internal/vm.h */
+#define rb_funcallv(...) rb_nonexistent_symbol(__VA_ARGS__)
+#define rb_method_basic_definition_p(...) rb_nonexistent_symbol(__VA_ARGS__)
 
 #endif /* RUBY_INTERNAL_H */