* configure.target: Add os_include_dir for mingw32* target.
* docs/html/17_intro/porting.texi: Mention mingw32 as target benefiting from
_GLIBCPP_AVOID_FSEEK.
* config/os/mingw32/bits/ctype_base.h: New file.
* config/os/mingw32/bits/ctype_inline.h: New file.
* config/os/mingw32/bits/ctype_noninline.h: New file.
* config/os/mingw32/bits/os_defines.h: New file.

From-SVN: r46857