FindOpenEXR.cmake: Add detection of debug builds

[Shootfast]

Description

Debug builds of OpenEXR have an _d suffix (on Windows at least). This patch adds these file names to the searched library names.

Tests

Built the code on Windows and linked against both Release and Debug builds of OpenEXR 2.4.1

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.