build: Fix substitution of MEEGO_PANELS_DIR (bgo#646544)
build: Fix substitution of MEEGO_PANELS_DIR (bgo#646544)

If meego-panel.pc is not found, substitute the old value of MEEGO_PANELS_DIR
instead, which was constructed from datadir. This is useful if you want
to use the Meego UI on non-Meego systems.

Signed-off-by: Bertrand Lorentz <eb7ac131ef7bff6d2ebccd6e5bf9a7cb6485cf2c@gmail.com>

Carbenium/banshee,ixfalia/banshee,Dynalon/banshee-osx,babycaseny/banshee,mono-soc-2011/banshee,arfbtwn/banshee,ixfalia/banshee,arfbtwn/banshee,mono-soc-2011/banshee,mono-soc-2011/banshee,GNOME/banshee,Dynalon/banshee-osx,babycaseny/banshee,Carbenium/banshee,Dynalon/banshee-osx,mono-soc-2011/banshee,arfbtwn/banshee,stsundermann/banshee,stsundermann/banshee,ixfalia/banshee,GNOME/banshee,ixfalia/banshee,stsundermann/banshee,ixfalia/banshee,Dynalon/banshee-osx,arfbtwn/banshee,dufoli/banshee,Dynalon/banshee-osx,arfbtwn/banshee,stsundermann/banshee,arfbtwn/banshee,stsundermann/banshee,GNOME/banshee,babycaseny/banshee,Dynalon/banshee-osx,dufoli/banshee,babycaseny/banshee,Carbenium/banshee,Carbenium/banshee,mono-soc-2011/banshee,Dynalon/banshee-osx,mono-soc-2011/banshee,dufoli/banshee,dufoli/banshee,dufoli/banshee,Dynalon/banshee-osx,GNOME/banshee,arfbtwn/banshee,GNOME/banshee,babycaseny/banshee,GNOME/banshee,stsundermann/banshee,Carbenium/banshee,babycaseny/banshee,dufoli/banshee,babycaseny/banshee,mono-soc-2011/banshee,babycaseny/banshee,GNOME/banshee,stsundermann/banshee,stsundermann/banshee,arfbtwn/banshee,dufoli/banshee,ixfalia/banshee,ixfalia/banshee,Carbenium/banshee,dufoli/banshee,ixfalia/banshee,GNOME/banshee