macro ( unset_pkg_config _prefix )
  unset ( ${_prefix}_VERSION CACHE )
  unset ( ${_prefix}_PREFIX CACHE )
  unset ( ${_prefix}_CFLAGS CACHE )
  unset ( ${_prefix}_CFLAGS_OTHER CACHE )
  unset ( ${_prefix}_LDFLAGS CACHE )
  unset ( ${_prefix}_LDFLAGS_OTHER CACHE )
  unset ( ${_prefix}_LIBRARIES CACHE )
  unset ( ${_prefix}_INCLUDEDIR CACHE )
  unset ( ${_prefix}_INCLUDE_DIRS CACHE )
  unset ( ${_prefix}_LIBDIR CACHE )
  unset ( ${_prefix}_LIBRARY_DIRS CACHE )
  unset ( __pkg_config_checked_${_prefix} CACHE )
endmacro ( unset_pkg_config )