#166 Build failure on OpenBSD 7.5

[FozzeY]

Compilation according to instructions on the wiki article fails on OpenBSD 7.5 with the following error:

In file included from SSPath.C:31:
In file included from ./SSPath.h:27:
In file included from ./SymTab.h:32:
In file included from ../dti_cc/cc_hdict.h:29:
In file included from ../dti_cc/cc_pvect.h:57:
In file included from ../dti_cc/cc_pvect.C:25:
In file included from ../dti_cc/cc_exceptions.h:28:
In file included from /usr/include/c++/v1/fstream:186:
In file included from /usr/include/c++/v1/__locale:18:
In file included from /usr/include/c++/v1/mutex:192:
In file included from /usr/include/c++/v1/__mutex_base:20:
In file included from /usr/include/c++/v1/system_error:154:
In file included from /usr/include/c++/v1/string:576:
In file included from /usr/include/c++/v1/string_view:1025:
In file included from /usr/include/c++/v1/algorithm:1920:
In file included from /usr/include/c++/v1/iterator:684:function-like macro invocation
In file included from /usr/include/c++/v1/__iterator/common_iterator.h:30:
/usr/include/c++/v1/variant:532:39: error: too few arguments provided to 
    return __at(__fmatrix, __vs.index()...)(
                                      ^
/usr/X11R6/include/X11/Xos.h:67:12: note: macro 'index' defined here
#   define index(s,c) (strchr((s),(c)))
           ^

There is a number of similar errors all indicating conflict with the index macro defined in Xos.h. When the macro definition is removed from the file, CDE compiles successfully. This is peculiar because this macro is seemingly defined in OpenBSD 7.4 as well, where I am able to build CDE without issues. Full build error log is attached.