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).

strftime.c

@@ -47,10 +47,7 @@
  * January 1996
  */
 
-#include "ruby/ruby.h"
-#include "ruby/encoding.h"
-#include "timev.h"
-#include "internal.h"
+#include "ruby/config.h"
 
 #ifndef GAWK
 #include <stdio.h>
@@ -68,6 +65,14 @@
 #endif
 #include <math.h>
 
+#include "internal.h"
+#include "internal/string.h"
+#include "internal/util.h"
+#include "internal/vm.h"
+#include "ruby/encoding.h"
+#include "ruby/ruby.h"
+#include "timev.h"
+
 /* defaults: season to taste */
 #define SYSV_EXT	1	/* stuff in System V ascftime routine */
 #define SUNOS_EXT	1	/* stuff in SunOS strftime routine */