The problem is from libsec256k1, not db48.
The general makefile is incomplete BTW, and I had to go into the secp256k1 and build it separately, however as the error seems to indicates, the version in the repository may not be the one used by the wallet (or it needs specific build options ?)
secp256k1 libs are a big mess with multiple incompatible versions, so there is a version of Linux dll hell when makefiles are not explicitly using static versions... could be what is happening here if the wallet is developed on a machine where another version of libsec256k1 was "installed".
Thank you for looking at it, we're focussed very much on making the network operate securely while we make these changes. We have compiled a windows wallet and we're testing the timelock feature in particular, so not finished yet by any means.
Your help is greatly appreciated.