rebuild tesseract if libpng or webp so version has changed.

component/tesseract/buildout.cfg

 [buildout]
 extends =
   ../automake/buildout.cfg
+  ../libpng/buildout.cfg
   ../libtool/buildout.cfg
   ../leptonica/buildout.cfg
   ../jbigkit/buildout.cfg
+  ../webp/buildout.cfg
 
 parts =
   tesseract
@@ -26,6 +28,9 @@ md5sum = 2e691858cb492b7c17d23bf0912b3d24
 recipe = slapos.recipe.cmmi
 url = http://tesseract-ocr.googlecode.com/files/tesseract-3.01.tar.gz
 md5sum = 1ba496e51a42358fb9d3ffe781b2d20a
+depends =
+  ${libpng:so_version}
+  ${webp:so_version}
 patch-options =
   -p1
 patches =