broken symlink libImath.so in debug build

[sboehmann]

When building OpenEXR 3.0.4 in debug mode, a symlink libImath.so is created pointing to libImath-3_0.so (which do not exist). However, this symlink should be called libImath_d.so and should point to libImath-3_0_d.so (similarly to libIlmThread_do.so where it is correct).

[cary-ilm]

Should be fixed by #1052 and https://github.com/AcademySoftwareFoundation/Imath/pull/166

[luke-titley]

I just hit this issue as well.