cmake: Find TBB dependency in Config.cmake.in when building static (#3557)

TBB::tbb is added as a library dependency via TARGET_NAME_IF_EXISTS.
Therefore if an executable links OpenImageIO static library without the
executable itself depending on TBB, then TBB will not be linked in and
result in link time errors.

Author: p12tic

src/cmake/Config.cmake.in

@@ -28,6 +28,11 @@ if (NOT @BUILD_SHARED_LIBS@)
     if (@PNG_FOUND@)
         find_dependency(PNG)
     endif()
+    # The following have the same problem except that INTERFACE_LINK_LIBRARIES use
+    # TARGET_NAME_IF_EXISTS, so the error only happens on link time.
+    if (@OIIO_TBB@)
+        find_dependency(TBB)
+    endif ()
 endif ()
 
 # Compute the installation prefix relative to this file. Note that cmake files are installed