Don't use __extension__ in C++ code

A few important notes:

*  __extension__ is a GNU C "alternate" keyword, not a C++ keyword.[1]

*  __extension__ is designed to work on "expressions"; it does work on
   #include_next in C mode, but it has no effect in C++ mode; the
   warning will still appear, if enabled, even with __extension__
   preceding #include_next.  This is because #include_next is not
   considered an expression in C++, so the compiler attaches
   __extension__ to the first expression of the header.

All of this leads us to a build failure while building at least all
Mozilla software.  Moz has an alternate -isystem dir searched before
/usr/include that overrides some headers, including <features.h>.  The
first statement in each of these headers is a #pragma, and since
__extension__ is looking for an expression, and #pragma is a "null"
expression, we end up with the following error:

dist/system_wrappers/features.h:1:9: error: '#pragma' is not allowed here

Since __extension__ has no effect on #include_next in C++ mode anyway,
and since it can cause breakage, this commit omits __extension__ in C++
mode.

[1]: https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Alternate-Keywords.html

include/poll.h

@@ -16,7 +16,9 @@
 #ifndef _FORTIFY_POLL_H
 #define _FORTIFY_POLL_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <poll.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0

include/stdio.h

@@ -16,7 +16,9 @@
 #ifndef _FORTIFY_STDIO_H
 #define _FORTIFY_STDIO_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <stdio.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0

include/stdlib.h

@@ -16,12 +16,16 @@
 #ifndef _FORTIFY_STDLIB_H
 #define _FORTIFY_STDLIB_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <stdlib.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0
 #if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <limits.h>
 #endif
 

include/string.h

@@ -16,7 +16,9 @@
 #ifndef _FORTIFY_STRING_H
 #define _FORTIFY_STRING_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <string.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0

include/sys/select.h

@@ -16,7 +16,9 @@
 #ifndef _FORTIFY_SYS_SELECT_H
 #define _FORTIFY_SYS_SELECT_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <sys/select.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0

include/sys/socket.h

@@ -16,7 +16,9 @@
 #ifndef _FORTIFY_SYS_SOCKET_H
 #define _FORTIFY_SYS_SOCKET_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <sys/socket.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0

include/unistd.h

@@ -16,7 +16,9 @@
 #ifndef _FORTIFY_UNISTD_H
 #define _FORTIFY_UNISTD_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <unistd.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0

include/wchar.h

@@ -16,13 +16,21 @@
 #ifndef _FORTIFY_WCHAR_H
 #define _FORTIFY_WCHAR_H
 
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <limits.h>
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <stdlib.h>
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <string.h>
+#ifndef __cplusplus
 __extension__
+#endif
 #include_next <wchar.h>
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0