Replace recursive calls to `make` with `$(MAKE)`

Ref: https://www.gnu.org/software/make/manual/make.html#Recursion
Build on Travis CI

Install `nettle-dev` even if there is logic in the build
infrastructure for downloading it because of build error for missing
`nettle/md5.h`.  Because of this, cannot use Travis CI container
(`sudo: false`).
Merge remote-tracking branch 'origin/ci' into HEAD