build: have a disable-static build succeed

When using configure --disable-static, no libosmogsm.a will be
created, and the tests fail to link because symbols like _a5_3 and
_a5_4 are not exported through the only remaining libosmogsm.so.

A method to overcome this is an intermediate private non-distributed
library, examples of which are present in e.g. libabc, kmod and
systemd.

With this, disable-static can now be the default and practical compile
time be halved.
3 files changed