FindHDF5: initialize HDF5_DEFINITIONS when using hdf5-config.cmake

Modules/FindHDF5.cmake

@@ -418,6 +418,7 @@ if(NOT HDF5_FOUND)
         set(HDF5_CXX_HL_TARGET ${_target_prefix}hdf5_hl_cpp)
         set(HDF5_Fortran_TARGET ${_target_prefix}hdf5_fortran)
         set(HDF5_Fortran_HL_TARGET ${_target_prefix}hdf5_hl_fortran)
+        set(HDF5_DEFINITIONS "")
         if(HDF5_USE_STATIC_LIBRARIES)
             set(_suffix "-static")
         else()