Build unison-2.51.2 deb, compatible with the MacOS binary release

make-deb.sh

#!/bin/bash

export DEBIAN_FRONTEND=noninteractive
apt-get update
apt-get -y install build-essential devscripts equivs git-buildpackage

cd /vagrant  # if using Vagrant (which you should ;)

wget https://github.com/bcpierce00/unison/archive/v2.51.2.tar.gz

git clone https://salsa.debian.org/ocaml-team/unison src
git config user.name "YOUR NAME"
git config user.email "you@example.com"

cd src
mk-build-deps -i debian/control -t 'apt-get --no-install-recommends -y'

git checkout upstream
git checkout master

# https://honk.sigxcpu.org/projects/git-buildpackage/manual-html/index.html
gbp import-orig ../v2.51.2.tar.gz
gbp dch --git-author --new-version=2.51.2-1 --release --auto --commit

patch -p1 < ../srcdir.diff
git commit -a -m 'Upstream introduced a src/ directory'

gbp buildpackage

srcdir.diff

commit c1a1c51e7aa6d3ec3175d9198e0587c63f2b6367
Author: Wolfgang Schnerring <wosc@wosc.de>
Date:   Tue Dec 18 18:23:11 2018 +0000

    Upstream introduced a src/ directory

diff --git a/debian/rules b/debian/rules
index 7b76a24..c981da1 100755
--- a/debian/rules
+++ b/debian/rules
@@ -80,9 +80,9 @@ endif
 
 UISTYLE := $(shell dpkg --compare-versions 2.10 ge $(VERSION) && echo gtk || echo gtk2)
 
-DEB_INSTALL_DOCS_ALL := BUGS.txt TODO.txt unison-manual.txt -X unison.doc-base.in
+DEB_INSTALL_DOCS_ALL := src/BUGS.txt src/TODO.txt unison-manual.txt -X unison.doc-base.in
 
-DEB_INSTALL_CHANGELOGS_ALL := NEWS
+DEB_INSTALL_CHANGELOGS_ALL := src/NEWS
 
 #
 # Build unison
@@ -92,19 +92,19 @@ configure::
 	cp config/Makefile.unix config/Makefile
 
 build/$(UNISON_PACKAGE)::
-	$(MAKE) UISTYLE=text NATIVE=$(NATIVE) $(MY_MAKE_FLAGS) CFLAGS=
-	mv '$(CURDIR)/unison' '$(CURDIR)/$(UNISON)'
+	$(MAKE) -C src UISTYLE=text NATIVE=$(NATIVE) $(MY_MAKE_FLAGS) CFLAGS=
+	mv '$(CURDIR)/src/unison' '$(CURDIR)/$(UNISON)'
 
 build/$(UNISON_GTK_PACKAGE):: debian/$(UNISON_GTK_PACKAGE).xpm
-	$(MAKE) UISTYLE=$(UISTYLE) NATIVE=$(NATIVE) $(MY_MAKE_FLAGS) CFLAGS=
-	mv '$(CURDIR)/unison' '$(CURDIR)/$(UNISON_GTK)'
+	$(MAKE) -C src UISTYLE=$(UISTYLE) NATIVE=$(NATIVE) $(MY_MAKE_FLAGS) CFLAGS=
+	mv '$(CURDIR)/src/unison' '$(CURDIR)/$(UNISON_GTK)'
 	if test 'x$(OCAML_HAVE_OCAMLOPT)' = 'xyes'; then /usr/bin/chrpath -d '$(CURDIR)/$(UNISON_GTK)'; fi
 
 clean::
 	-$(RM) '$(CURDIR)/$(UNISON_GTK)'
 	-$(RM) '$(CURDIR)/$(UNISON)'
-	$(MAKE) clean
-	rm lwt/depend ubase/depend
+	$(MAKE) -C src clean
+	rm src/lwt/depend src/ubase/depend
 
 debian/unison-manual.txt: build/$(UNISON_PACKAGE)
 	env HOME='$(CURDIR)' '$(CURDIR)/$(UNISON)' -doc all > '$(CURDIR)/unison-manual.txt'

Vagrantfile

Vagrant.configure("2") do |config|
  config.vm.define "default" do |default|
    default.vm.hostname = "unisonbuild"
    default.vm.provider "lxc"
    default.vm.box = "zeitonline/bionic64-lxc"
    # default.vm.box = "bento/ubuntu-18.04"
  end
end

Thank you!