Build of ufo-filters fails if both GTK_DOC and OpenCV are enabled

[Suren A. Chilingaryan]

Build of ufo-filters fails if both GTK_DOC and OpenCV are enabled.
branch: master
revision: fd4b2c0814d1979362c303f2e07ad58bd964ff2b

[ 89%] Generating ../docs/reference/UfoFilters.args
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvDestroyWindow'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvEllipse'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvReleaseImage'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvFree_'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvRead'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvShowImage'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvSeqSlice'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvGetCols'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvSetAdd'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvGetSeqElem'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvInitFont'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvArcLength'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvConvertImage'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvGetFileNodeByName'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvReleaseMat'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvMoveWindow'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvGetRows'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvWaitKey'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvNamedWindow'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvCalcArrHist'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvCreateImageHeader'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvAddS'
/root/ufo-filters/src/libufofiltercvshow.so: undefined reference to `cvCreateImage'
collect2: error: ld returned 1 exit status
Linking of scanner failed: 
make[2]: *** [src/../docs/reference/UfoFilters.args] Error 1
make[1]: *** [src/CMakeFiles/reference.dir/all] Error 2
make: *** [all] Error 2

[Matthias Vogelgesang]

At least on ufosrv1, this does not happen anymore.