empty vs notfound

[David Faure]

FindFontconfig says

  if (FONTCONFIG_LIBRARIES)
     set(FONTCONFIG_FOUND TRUE)
  endif (FONTCONFIG_LIBRARIES)

and doesn't test FONTCONFIG_INCLUDE_DIR because it's ok if that one is empty, as happens
when fontconfig is in /usr/include (right?)

However this breaks when it's set to FONTCONFIG_INCLUDE_DIR-NOTFOUND,
i.e. when the headers are not found at all - then FONTCONFIG_FOUND is still set
to TRUE, and the use of FONTCONFIG_INCLUDE_DIR in cmakelists.txt leads to an error.

How can we check that FONTCONFIG_INCLUDE_DIR is "set, or empty, but not NOTFOUND" ?

-- 
David Faure, faure at kde.org, sponsored by Trolltech to work on KDE,
Konqueror (http://www.konqueror.org), and KOffice (http://www.koffice.org).