doc: QtGL: use pkg-config to find libvlc

The sample can now be built using either directly `qmake` (using
pkg-config to find the library on the system) or with the longer
`PKG_CONFIG_PATH=/path/where/vlc/is/installed qmake`.

doc/libvlc/QtGL/QtGl.pro

 TEMPLATE = app
 TARGET = qtglvlc
-DEPENDPATH += .
-INCLUDEPATH += . ../../../include
-LIBS += -lvlc
-#-L../../../../build/git64/lib/.libs
+
+CONFIG += link_pkgconfig
+PKGCONFIG = libvlc
 QT += widgets
 
 SOURCES += main.cpp qtvlcwidget.cpp