buildroot

mpd: update to use the gcc version dependency mechanism

mpd requires at least gcc 4.6, so use the newly introduced gcc version
dependency mechanism instead of open-coded toolchain dependencies.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>