openexr: Fix check for when to pass -std=c++98

commit 6adfb120 added this flag to fix builds with -Werror, and afterwards it was changed to use a version check when newer versions of openexr moved over to C++11.

However, some distros have backported patches to older openexr versions which make it require C++11, which makes the version check incorrect and causes an error because we passed -Werror -std=c++98.

Instead, directly check when usage of the header requires -std=c++98 with -Werror and override the cpp_std setting on the target.

Fixes #1117 (closed)