Fix non-versioned library symlinks in debug build.

[mcmtroffaes]

Follow-up from discussion at #1032.

Old behaviour for debug builds: libOpenEXR.so -> libOpenEXR-3_1.so (broken, target should be libOpenEXR-3_1_d.so)
New behaviour for debug builds: libOpenEXR_d.so -> libOpenEXR-3_1_d.so

(Also needs fixing in RB-2.5 and Imath, will follow that up once this is merged.)

[mcmtroffaes]

One thing I'm not sure of is whether the test (NOT "${OPENEXR_LIB_SUFFIX}" STREQUAL "") should instead read (NOT "${OPENEXR_LIB_SUFFIX}${CMAKE_${uppercase_CMAKE_BUILD_TYPE}_POSTFIX}" STREQUAL ""). My gut feeling says "yes" but I thought it prudent to ask before adding it in.

EDIT: Scrap that comment... I think it's fine as it is, it seems it's just a guard to prevent symlink source and target to be identical: if ${OPENEXR_LIB_SUFFIX} is empty but ${CMAKE_${uppercase_CMAKE_BUILD_TYPE}_POSTFIX} is not, the link should not be created.

[meshula]

Looks good, thanks!