upmpdcli: update to use the gcc version dependency mechanism

This commit updates the upmpdcli Config.in file to use the newly
introduced gcc version dependency mechanism to depend on gcc >= 4.6
instead of open-coding dependencies on specific toolchains.

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