(but keep it conditional on __ISO_C_VISIBLE >= 1999.
Why? Our out /usr/src/contrib assumes it, and more than a few ports have
an autoconf that looks for __va_copy because it is available on glibc.
It is critical that we use it on PowerPC. It generally isn't a problem
for i386 and its ilk because those platforms can get away with cheating
the C standard, using a plain assignment.
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