Make sure -fPIC is passed to ExternalProject VORO if BUILD_SHARED_LIBS=on

  option(DOWNLOAD_VORO "Download voro++ (instead of using the system's one)" OFF)

  if(DOWNLOAD_VORO)

    include(ExternalProject)

    if(BUILD_SHARED_LIBS)

        set(VORO_BUILD_OPTIONS "CFLAGS=-fPIC")

    else()

        set(VORO_BUILD_OPTIONS "")

    endif()

    ExternalProject_Add(voro_build

      URL http://math.lbl.gov/voro++/download/dir/voro++-0.4.6.tar.gz

      URL_MD5 2338b824c3b7b25590e18e8df5d68af9

      CONFIGURE_COMMAND "" BUILD_IN_SOURCE 1 INSTALL_COMMAND "" 

      CONFIGURE_COMMAND "" BUILD_COMMAND make ${VORO_BUILD_OPTIONS} BUILD_IN_SOURCE 1 INSTALL_COMMAND ""

      )

    ExternalProject_get_property(voro_build SOURCE_DIR)

    set(VORO_LIBRARIES ${SOURCE_DIR}/src/libvoro++.a)