Configure can't find open3 on NeXTstep

Configure can't find open3 on NeXTstep

A buglet in Configure casuses it to not find the 3-argument
form of open(2) on some platforms.  The problem is that it
tries to compile open3.c using $cc $cppflags.  On a NeXT
with MAB support, this will cross-compile for the first
architecture on the list.  If that isn't the architecture
Configure is running on, the resultant binary cannot be run.
This leads Configure to believe that open3 is not available.

P.S. A relevant detail not mentioned in the site configurion
summary below, is that I'm compiling on an HP/PA workstation.

Credited: Andy Dougherty <doughera@newton.phys.lafayette.edu>

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