assume C99

Now that we no longer support old compilers, we can safely delete
several obsolete #ifdef gurads.  Also because (as of writing) it is
impossible to compile the program using C++ compilers, lets just
entirely prohibit __cplusplus to reduce # of LOCs.

Note however that we still cannot eliminate __STDC_VERSION__ checks,
because MSVC does not define it, saying its C99 support is partial.
See also https://social.msdn.microsoft.com/Forums/vstudio/en-US/53a4fd75-9f97-48b2-aa63-2e2e5a15efa3

internal.h

@@ -12,13 +12,10 @@
 #ifndef RUBY_INTERNAL_H
 #define RUBY_INTERNAL_H 1
 
-#include "ruby.h"
+#include "ruby/config.h"
 
-#if defined(__cplusplus)
-extern "C" {
-#if 0
-} /* satisfy cc-mode */
-#endif
+#ifdef __cplusplus
+# error not for C++
 #endif
 
 #include "ruby/encoding.h"
@@ -87,10 +84,4 @@ extern "C" {
 #include "internal/inits.h"
 #include "internal/warnings.h"
 
-#if defined(__cplusplus)
-#if 0
-{ /* satisfy cc-mode */
-#endif
-}  /* extern "C" { */
-#endif
 #endif /* RUBY_INTERNAL_H */

internal/serial.h

@@ -10,21 +10,13 @@
  *             file COPYING are met.  Consult the file for details.
  */
 
-#if defined(HAVE_LONG_LONG)
-typedef unsigned LONG_LONG rb_serial_t;
-# define SERIALT2NUM ULL2NUM
-# define PRI_SERIALT_PREFIX PRI_LL_PREFIX
-# define SIZEOF_SERIAL_T SIZEOF_LONG_LONG
-#elif defined(HAVE_UINT64_T)
-typedef uint64_t rb_serial_t;
-# define SERIALT2NUM SIZET2NUM
-# define PRI_SERIALT_PREFIX PRI_64_PREFIX
-# define SIZEOF_SERIAL_T SIZEOF_UINT64_T
-#else
-typedef unsigned long rb_serial_t;
-# define SERIALT2NUM ULONG2NUM
-# define PRI_SERIALT_PREFIX PRI_LONG_PREFIX
-# define SIZEOF_SERIAL_T SIZEOF_LONG
+#ifndef HAVE_LONG_LONG
+# error need C99+
 #endif
 
+typedef unsigned LONG_LONG rb_serial_t;
+#define SERIALT2NUM ULL2NUM
+#define PRI_SERIALT_PREFIX PRI_LL_PREFIX
+#define SIZEOF_SERIAL_T SIZEOF_LONG_LONG
+
 #endif /* INTERNAL_SERIAL_H */