Enable optional static builds
Enable optional static builds

The default (for both manual and .deb builds) is to use shared build (as
before) - the static build is entirely optional.

Change-Id: Iabdebefef5c07dd1cd4b94b29ca40c6be0f8adda

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts