go on only if pkg-config says that all is fine

svn path=/branches/work/kde4/playground/graphics/cmake/; revision=550781

cmake/modules/modules/FindDjVuLibre.cmake

@@ -16,6 +16,7 @@ else (DJVULIBRE_INCLUDE_DIR AND DJVULIBRE_LIBRARY)
 
   pkgconfig(ddjvuapi _ddjvuIncDir _ddjvuLinkDir ddjvuLinkFlags _ddjvuCflags)
 
+  if(_ddjvuIncDir)
     find_path(DJVULIBRE_INCLUDE_DIR libdjvu/ddjvuapi.h
       ${_ddjvuIncDir}
       /usr/local/include
@@ -30,6 +31,7 @@ else (DJVULIBRE_INCLUDE_DIR AND DJVULIBRE_LIBRARY)
       /usr/local/lib
       ${GNUWIN32_DIR}/lib
     )
+  endif(_ddjvuIncDir)
 
   if(DJVULIBRE_INCLUDE_DIR AND DJVULIBRE_LIBRARY)
     set(DJVULIBRE_FOUND TRUE)