CMake Visibility for Imported Libraries - c++

Question

I have an imported library that I am trying to link using CMake. However, the library always shows up empty (or not found). I know that imported libraries have different visibility rules (according to this thread) than the standard libraries, but no matter what I try the library always ends up missing. To start, here's my folder structure:
Root
├── build
├── cmake
├── src
│ ├── external
│ │ ├── openvdb
│ │ │ ├── openvdb
│ │ │ ├── cmake
│ │ │ ├── CMakeLists.txt
│ │ ├── tbb
│ │ ├── zlib
│ │ ├── CMakeLists.txt
│ ├── CMakeLists.txt
├── CMakeLists.txt
In my Root CMakeLists.txt file I add the libraries called TBB_LIBS and TBBMALLOC_LIBS like this:
cmake_minimum_required(VERSION 3.2.2 FATAL_ERROR)
include(ExternalProject)
project(openVDBBuild)
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
if (WIN32)
set(CMAKE_CXX_FLAGS "-bigobj /EHsc")
endif ()
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/lib")
if(NOT CMAKE_DEBUG_POSTFIX)
set(CMAKE_DEBUG_POSTFIX debug)
endif()
if(NOT CMAKE_RELEASE_POSTFIX)
set(CMAKE_RELEASE_POSTFIX release)
endif()
set(BUILD_VARIANTS "release debug")
set(TBB_PREFIX "tbb")
set(TBB_LIBS_DIR "${CMAKE_SOURCE_DIR}/src/external/tbb/build/${TBB_PREFIX}_")
set(TBB_ROOT ${CMAKE_SOURCE_DIR}/src/external/tbb)
set(TBB_INCLUDE_DIR ${TBB_ROOT}/include)
#********** Here I create the library and add the Global Flag ***********
add_library(TBBMALLOC_LIBS SHARED IMPORTED GLOBAL)
add_library(TBB_LIBS SHARED IMPORTED GLOBAL)
if (WIN32)
#********** Here I set different paths for debug and release versions of the library ***********
set_property(TARGET TBBMALLOC_LIBS PROPERTY IMPORTED_LOCATION_RELEASE ${CMAKE_BINARY_DIR}/lib/tbbmalloc.dll )
set_property(TARGET TBBMALLOC_LIBS PROPERTY IMPORTED_LOCATION_DEBUG ${CMAKE_BINARY_DIR}/lib/tbbmalloc_${CMAKE_DEBUG_POSTFIX}.dll )
set_property(TARGET TBB_LIBS PROPERTY IMPORTED_LOCATION_RELEASE ${CMAKE_BINARY_DIR}/lib/tbb.dll )
set_property(TARGET TBB_LIBS PROPERTY IMPORTED_LOCATION_DEBUG ${CMAKE_BINARY_DIR}/lib/tbb_${CMAKE_DEBUG_POSTFIX}.dll )
add_custom_command(
TARGET ProjectTbbExternal
POST_BUILD
COMMAND ${CMAKE_COMMAND} -E copy "$<$<CONFIG:debug>:${TBB_LIBS_DIR}${CMAKE_DEBUG_POSTFIX}/tbb_${CMAKE_DEBUG_POSTFIX}.dll>$<$<CONFIG:release>:${TBB_LIBS_DIR}${CMAKE_RELEASE_POSTFIX}/tbb.dll>" "${CMAKE_BINARY_DIR}/lib"
COMMAND ${CMAKE_COMMAND} -E copy "$<$<CONFIG:debug>:${TBB_LIBS_DIR}${CMAKE_DEBUG_POSTFIX}/tbbmalloc_${CMAKE_DEBUG_POSTFIX}.dll>$<$<CONFIG:release>:${TBB_LIBS_DIR}${CMAKE_RELEASE_POSTFIX}/tbbmalloc.dll>" "${CMAKE_BINARY_DIR}/lib"
COMMAND ${CMAKE_COMMAND} -E copy "$<$<CONFIG:debug>:${TBB_LIBS_DIR}${CMAKE_DEBUG_POSTFIX}/tbbmalloc_proxy_${CMAKE_DEBUG_POSTFIX}.dll>$<$<CONFIG:release>:${TBB_LIBS_DIR}${CMAKE_RELEASE_POSTFIX}/tbbmalloc_proxy.dll>" "${CMAKE_BINARY_DIR}/lib"
)
endif ()
add_subdirectory("src")
Now, down inside the CMakeLists.txt in the src/external/openvdb directory I try to link the libraries like this:
project(OpenVDBCore)
#define OPENVDB_LIBRARY_MAJOR_VERSION_NUMBER 4
#define OPENVDB_LIBRARY_MINOR_VERSION_NUMBER 0
#define OPENVDB_LIBRARY_PATCH_VERSION_NUMBER 0
#*********** I'm going to omit some lines here since **********
#*********** they're standard for the OpenVDB build ***********
SET_SOURCE_FILES_PROPERTIES ( ${OPENVDB_LIBRARY_SOURCE_FILES}
PROPERTIES
COMPILE_FLAGS "-DOPENVDB_PRIVATE -DOPENVDB_USE_BLOSC ${OPENVDB_USE_GLFW_FLAG}"
)
ADD_LIBRARY ( openvdb_static STATIC ${OPENVDB_LIBRARY_SOURCE_FILES} )
ADD_LIBRARY ( openvdb_shared SHARED ${OPENVDB_LIBRARY_SOURCE_FILES} )
TARGET_LINK_LIBRARIES ( openvdb_static
zlib
TBB_LIBS
TBBMALLOC_LIBS
)
TARGET_LINK_LIBRARIES ( openvdb_shared
zlib
TBB_LIBS
TBBMALLOC_LIBS
)
MESSAGE(STATUS "TBB_LIBS library location is ${TBB_LIBS}")
MESSAGE(STATUS "TBBMALLOC_LIBS library location is ${TBBMALLOC_LIBS}")
You can see that I put a status message in CMake to see what the ${TBB_LIBS} and ${TBBMALLOC_LIBS} variables are... but that part is always empty. Accordingly, the openvdb_shared and openvdb_static project files that get generated by CMake also do not link the libraries correctly (since those libraries are unknown). Does anyone know why this occurs (and how to fix it)?