Error when building shared libs on Windows.

[JenusL]

Hi!
When building OCIO with shared libs on windows it fails to link correctly with Imath.
I get the following when building with Imath 3.1.10, OpenExr 3.2.1 and OpenColorIO 2.3.2

Lut1DOpData.obj : error LNK2001: unresolved external symbol imath_half_to_float_table
OpHelpers.obj : error LNK2001: unresolved external symbol imath_half_to_float_table
CPUProcessor.obj : error LNK2001: unresolved external symbol imath_half_to_float_table
FileFormatDiscreet1DL.obj : error LNK2001: unresolved external symbol imath_half_to_float_table
MathUtils.obj : error LNK2001: unresolved external symbol imath_half_to_float_table
Lut1DOpCPU.obj : error LNK2001: unresolved external symbol imath_half_to_float_table
..\vfxplatform_build\build\OpenColorIO\src\OpenColorIO\Release\OpenColorIO_2_3.dll : fatal error LNK1120: 1 unresolved externals

This seem to be a common issue with Imath and I think the solution is to define IMATH_DLL when needed, but I'm not sure where or with what logic it should be defined, but I guess only when building shared on Windows.
In my test to see if I could work around it I just added the define to FindImath.cmake and then it builds correctly.
More info in this Imath ticket AcademySoftwareFoundation/Imath#212 (comment)

[kmilos]

FYI, recent Imath ships with its own CMake config files which should already take care of adding the IMATH_DLL compile definition to the imported Imath::Imath target. E.g. on MinGW I have this in lib/cmake/Imath/ImathTargets.cmake:

set_target_properties(Imath::Imath PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "IMATH_DLL"
  INTERFACE_COMPILE_FEATURES "cxx_std_11"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
  INTERFACE_LINK_LIBRARIES "Imath::ImathConfig;m"
)

So this could be MSVC specific, as there is no intervention needed to build OCIO w/ shared Imath on MinGW.