more improvements to CMake build for USER-PLUMED on MacOSX

if(PKG_USER-PLUMED)

  find_package(GSL REQUIRED)

  if(APPLE)

    set(PLUMED_MODE "shared" CACHE STRING "Linkage mode for Plumed2 library")

  else()

    set(PLUMED_MODE "static" CACHE STRING "Linkage mode for Plumed2 library")

  endif()

  set(PLUMED_MODE_VALUES static shared runtime)

  set_property(CACHE PLUMED_MODE PROPERTY STRINGS ${PLUMED_MODE_VALUES})

  validate_option(PLUMED_MODE PLUMED_MODE_VALUES)

  string(TOUPPER ${PLUMED_MODE} PLUMED_MODE)

  if((PLUMED_MODE STREQUAL "STATIC") AND NOT LAPACK_LIBRARIES)

  set(PLUMED_LINK_LIBS "")

  if(PLUMED_MODE STREQUAL "STATIC")

    find_package(LAPACK REQUIRED)

    find_package(BLAS REQUIRED)

    find_package(GSL REQUIRED)

    list(APPEND LAPACK_LIBRARIES ${BLAS_LIBRARIES})

    list(APPEND PLUMED_LINK_LIBS ${LAPACK_LIBRARIES} ${GSL_LIBRARIES})

    find_package(ZLIB QUIET)

    if(ZLIB_FOUND)

      list(APPEND PLUMED_LINK_LIBS ${ZLIB_LIBRARIES})

    endif()

  endif()

  find_package(PkgConfig QUIET)

    list(APPEND LAMMPS_DEPS plumed_build)

    if(PLUMED_MODE STREQUAL "STATIC")

      add_definitions(-D__PLUMED_WRAPPER_CXX=1)

      list(APPEND LAMMPS_LINK_LIBS ${PLUMED_INSTALL_DIR}/lib/libplumed.a ${GSL_LIBRARIES} ${LAPACK_LIBRARIES} ${CMAKE_DL_LIBS})

      list(APPEND LAMMPS_LINK_LIBS ${PLUMED_INSTALL_DIR}/lib/libplumed.a ${PLUMED_LINK_LIBS} ${CMAKE_DL_LIBS})

    elseif(PLUMED_MODE STREQUAL "SHARED")

      list(APPEND LAMMPS_LINK_LIBS ${PLUMED_INSTALL_DIR}/lib/libplumed${CMAKE_SHARED_LIBRARY_SUFFIX} ${PLUMED_INSTALL_DIR}/lib/libplumedKernel${CMAKE_SHARED_LIBRARY_SUFFIX} ${CMAKE_DL_LIBS})

    elseif(PLUMED_MODE STREQUAL "RUNTIME")