Hi,
I have installed a recent libosmocore before I tried to compile your testing branch.
Did you also do a fresh checkout or a "git clean -f -x -d" when switching branch ?
Due to changes in the build system, residual from an old compilation may prevent it from building.
In the mean time you can find instructions to install libosmocore in https://bs11-abis.gnumonks.org/trac/wiki/Building_OpenBSC
I try to read the instructions above, but I haven't access to the server because no user and password.
Remove the 'https' ... it's only for authenticated wiki users.
Cheers,
Sylvain