opencv_world doesn't respect HIGHGUI_LIBRARY_DIRS (Bug #3046)

opencv_higui use pkg-config to find third party modules, and add them to HIGHGUI_LIBRARY_DIRS
but opencv_world ignore HIGHGUI_LIBRARY_DIRS, so these may lead to opencv_world link to different third party library.

for example, i have ffmpeg in /usr/local/lib as well as /another/dir, and i can override PKG_CONFIG_EXECUTABLE to force use
ffmpeg from /another/dir. But opencv_world still link to ffmpeg from /usr/local/lib since it does not add the proper
-L/another/dir option to the linker.

i think this cause two problems:
1) opencv_world may link with different third party libraries compared with other opencv modules;
2) we can not set preferred third party libraries with opencv_world