COMP: Fix Assimp linking on Ubuntu

https://github.com/assimp/assimp/issues/746
https://stackoverflow.com/questions/38296756/what-is-the-idiomatic-way-in-cmake-to-add-the-fpic-compiler-option

CMake/External/External_Assimp.cmake

@@ -7,6 +7,7 @@ imstk_add_external_project( Assimp
   GIT_TAG v3.3.1
   INSTALL_COMMAND ${SKIP_STEP_COMMAND}
   CMAKE_CACHE_ARGS
+    -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON
     -DASSIMP_BUILD_ASSIMP_TOOLS:BOOL=OFF
     -DASSIMP_BUILD_TESTS:BOOL=OFF
     -DASSIMP_NO_EXPORT:BOOL=ON