Add liblz4-dev to the Build-Depends. In addition you will need to patch
src/bin/Makefile.am and src/bin/termptysave.c so that the system version
is used. Since you are patching the autotools build system you will need
to replace autotools-dev with dh-autoreconf in the Build-Depends and
replace autotools-dev with autoreconf in debian/rules.