guards for dependency targets

cmake/HighFiveOptionalDependencies.cmake

@@ -1,24 +1,24 @@
-if(HIGHFIVE_USE_BOOST)
+if(HIGHFIVE_USE_BOOST AND NOT TARGET HighFiveBoostDependency)
   add_library(HighFiveBoostDependency INTERFACE)
   find_package(Boost REQUIRED)
   target_link_libraries(HighFiveBoostDependency INTERFACE Boost::headers)
   # TODO check if we need Boost::disable_autolinking to cause:
   # -DBOOST_ALL_NO_LIB (does something on MSVC).
 endif()
 
-if(HIGHFIVE_USE_EIGEN)
+if(HIGHFIVE_USE_EIGEN AND NOT TARGET HighFiveEigenDependency)
   add_library(HighFiveEigenDependency INTERFACE)
   find_package(Eigen3 REQUIRED NO_MODULE)
   target_link_libraries(HighFiveEigenDependency INTERFACE Eigen3::Eigen)
 endif()
 
-if(HIGHFIVE_USE_XTENSOR)
+if(HIGHFIVE_USE_XTENSOR AND NOT TARGET HighFiveXTensorDependency)
   add_library(HighFiveXTensorDependency INTERFACE)
   find_package(xtensor REQUIRED)
   target_link_libraries(HighFiveXTensorDependency INTERFACE xtensor)
 endif()
 
-if(HIGHFIVE_USE_OPENCV)
+if(HIGHFIVE_USE_OPENCV AND NOT TARGET HighFiveOpenCVDependency)
   add_library(HighFiveOpenCVDependency INTERFACE)
   find_package(OpenCV REQUIRED)
   target_include_directories(HighFiveOpenCVDependency SYSTEM INTERFACE ${OpenCV_INCLUDE_DIRS})