Fix libigl-config.cmake.in (#2188)

cmake/igl/libigl-config.cmake.in

@@ -1,3 +1,7 @@
 @PACKAGE_INIT@
 
+include(CMakeFindDependencyMacro)
+find_dependency(Eigen3 REQUIRED)
+find_dependency(Threads REQUIRED)
+include("${CMAKE_CURRENT_LIST_DIR}/LibiglConfigTargets.cmake")
 check_required_components(Libigl)