Make local build with Imath (AcademySoftwareFoundation#1219)

```
add_subdirectory(${IMATH_ROOT} Imath)
add_subdirectory(${OPENEXR_ROOT} OpenEXR)
```

Signed-off-by: Timothy Lyanguzov <timothy.lyanguzov@sap.com>

cmake/OpenEXRSetup.cmake

@@ -291,4 +291,14 @@ if(NOT TARGET Imath::Imath AND NOT Imath_FOUND)
   endif()
 else()
   message(STATUS "Using Imath from ${Imath_DIR}")
+  # local build
+  # add_subdirectory(${IMATH_ROOT} Imath)
+  # add_subdirectory(${OPENEXR_ROOT} OpenEXR)
+  if(NOT TARGET Imath::ImathConfig AND TARGET Imath AND TARGET ImathConfig)
+    get_target_property(imathinc Imath INTERFACE_INCLUDE_DIRECTORIES)
+    get_target_property(imathconfinc ImathConfig INTERFACE_INCLUDE_DIRECTORIES)
+    list(APPEND imathinc ${imathconfinc})
+    set(IMATH_HEADER_ONLY_INCLUDE_DIRS ${imathinc})
+    message(STATUS "Imath interface dirs ${IMATH_HEADER_ONLY_INCLUDE_DIRS}")
+  endif()
 endif()