graphics/openexr: use official WITH_DEBUG=yes fix

We've developed an officially accepted solution with upstream and
upstream reporter of the issue that prompted the change that broke our
DEBUG builds, so let's switch to the official change to minimize
differences from upstream.

https://github.com/AcademySoftwareFoundation/openexr/issues/1981
https://github.com/AcademySoftwareFoundation/openexr/pull/2166/commits/efa3b861ab0ab515ec7923ce02500acf667eabb0

PR: 290615

Note I am not updating to v3.4.2 because it does not contain changes
for FreeBSD - it contains a fix for Windows, and the other Imath change
is unnecessary for us because our ports system handles the requisite.