Require OpenEXR 2.3.0 at least, and do not force C++98
OpenEXR 2.4.0 requires at least C++11 (and supports C++14/17), so drop the extra compiler flags. C++11 support was added with OpenEXR 2.3.0.
OpenEXR 2.3.0 was released more than a year ago, and contains many correctness and security fixes, even more so the just released 2.4.0.
Also see #1089 (closed)