oils-for-unix build failures on OpenBSD due to stdin macro conflict

[andychu]

And there are multiple errors caused by stdin/stdout/stderr being macros:

_gen/bin/oils_for_unix.mycpp.cc:5888:22: error: '__sF' declared as array of references of type 'mylib::LineReader *(&)'
  mylib::LineReader* stdin;
                     ^
/usr/include/stdio.h:197:21: note: expanded from macro 'stdin'
#define stdin   (&__sF[0])
                      ^
_gen/bin/oils_for_unix.mycpp.cc:5896:37: error: expected identifier
         | maskbit_v(offsetof(Read, stdin));
                                    ^
/usr/include/stdio.h:197:15: note: expanded from macro 'stdin'
#define stdin   (&__sF[0])
                ^
...etc...

[andychu]

Released - https://www.oilshell.org/blog/2023/05/release-0.15.0.html