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

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