Thanks for the feedback. I am unable to reproduce this error anywhere but within TravisCI itself. I have tried the following and all work:
- Building on a personal Linux machine.
- Building on a personal macOS machine.
- Building in Docker against the TravisCI Ubuntu 16.04 docker image from dockerhub.
All of these work and the library is linked correctly. I see in the output of the Makefile config that the library is found:
checking for -lgmp... yes
checking for secp256k1_recovery.h... no
checking for secp256k1_ecdh.h... no
creating Makefile
All of this is quite puzzling. Especially since this build worked for quite a long time and then started breaking all of a sudden. So my guess is something has changed specifically on TravisCI.