headers: Drop _G_config.h from public headers.

Adjust the two legacy stdc++ headers that still included it;
it seems they didn't themselves depend on any of the definitions it
contained. However, streambuf.h actually did, so put that definition in
libio.h so that it's always present.

(The stdc++ implementation will get the 'real' _G_config.h from
libroot glibc.)

headers/cpp/cmath

@@ -3,7 +3,6 @@
 
 #ifndef __CMATH__
 #define __CMATH__
-#include <_G_config.h>
 #include <math.h>
 
 #ifdef __GNUG__

headers/cpp/libio.h

@@ -81,6 +81,7 @@
 #define _IO_IS_APPENDING 0x1000
 #define _IO_IS_FILEBUF 0x2000
 #define _IO_BAD_SEEN 0x4000
+#define _G_IO_IO_FILE_VERSION 0x20001
 
 /* These are "formatting flags" matching the iostream fmtflags enum values. */
 #define _IO_SKIPWS 01

headers/cpp/stl_config.h

@@ -161,7 +161,6 @@
 # endif
 
 # ifdef __GNUC__
-#   include <_G_config.h>
 #   define __STL_HAS_WCHAR_T
 #   if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 8)
 #     define __STL_STATIC_TEMPLATE_MEMBER_BUG

headers/cpp/streambuf.h

@@ -55,12 +55,8 @@ extern "C" {
 #endif
 
 #ifndef _IO_wchar_t
-#if _G_IO_IO_FILE_VERSION == 0x20001
-#define _IO_wchar_t _G_wchar_t
-#else
 #define _IO_wchar_t wchar_t
 #endif
-#endif
 
 extern "C++" {
 class istream; /* Work-around for a g++ name mangling bug. Fixed in 2.6. */