Compiling C++ code with `-D_XOPEN_SOURCE=600` causes problems on
OpenBSD. If that define is set, the C++ standard header detection
routine in BDB's configure script fails. This results in
`HAVE_CXX_STDHEADERS` not being defined, which then it turn leads to
the inclusion of `<iostream.h>` (rather than `<iostream>`), which
doesn't exist.
According to a mailing list post discussing a similar problem [1],
"OpenBSD provides the POSIX APIs by default", so we don't need this
define anyway and can remove it. This fixes the BDB build problem as
described in issue #28963.
Tested on OpenBSD 7.4 with clang 13.0.0.
[1] https://www.mail-archive.com/tech@openbsd.org/msg63386.html
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