FindPkgConfig: correct library search path for imported targets.

-L in LDFLAGS doesn't mean that standard search paths should be excluded. Example:

$ pkg-config --libs libmutter-2
-L/usr/lib/mutter -lmutter-2 -lgtk-3 -lgdk-3 -lmutter-clutter-2 -lcairo-gobject -latk-1.0 -lpangocairo-1.0 -lpango-1.0 -lfribidi -ljson-glib-1.0 -lgio-2.0 -lwayland-egl -lwayland-client -lXtst -lXi -lmutter-cogl-2 -lcairo -lgmodule-2.0 -pthread -lgdk_pixbuf-2.0 -lgobject-2.0 -lglib-2.0 -lgbm -ldrm -lwayland-server -lEGL -lXext -lXdamage -lXfixes -lXcomposite -lXrandr -lX11

Most of these libraries are located at standard paths, not in /usr/lib/mutter

Fixes: #17954 (closed)