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
/usr/include/fortify/string.h: In function 'void* __fortify_memcpy(void*, const void*, size_t)':
/usr/include/fortify/string.h:13:12: error: invalid conversion from 'void*' to 'char*' [-fpermissive]
/usr/include/fortify/string.h:14:18: error: invalid conversion from 'const void*' to 'const char*' [-fpermissive]
Since we are relying on GCC anyway, assume void * arithmetic is OK.
Do not perform checks only when `n' is constant, most of the time
it is not.
The bos == (size_t)-1 check is redundant because n > bos
with bos == -1 will always be false.
