This replaces uses of BUFSIZ with a new _GLIBCXX_BUFSIZ macro that can
be overridden in target-specific config headers.
That allows the mingw and mingw-w64 targets to override it, because
BUFSIZ is apparently defined to 512, resulting in poor performance. The
MSVCRT stdio apparently uses 4096, so we use that too.
libstdc++-v3/ChangeLog:
PR libstdc++/94268
* config/os/mingw32-w64/os_defines.h (_GLIBCXX_BUFSIZ):
Define.
* config/os/mingw32/os_defines.h (_GLIBCXX_BUFSIZ):
Define.
* include/bits/fstream.tcc: Use _GLIBCXX_BUFSIZ instead
of BUFSIZ.
* include/ext/stdio_filebuf.h: Likewise.
* include/std/fstream (_GLIBCXX_BUFSIZ): Define.
0bc199fc5d