libupnpp: update to use the gcc version dependency mechanism

This commit changes the libupnp Config.in to use the gcc version
dependency mechanism. The only reverse dependency of libupnpp is
upmpdcli, which has already been updated, and requires >= 4.6, while
libupnpp only requires >= 4.5.

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