Add a Makefile with a few targets. The default is 'build'. The build

target calls the new version-at-commit.sh script which will
automatically populate the version variable inside the Headscale binary.

Once we start tagging releases on the git tree, that will come in handy.

The Makefile also has a 'test' target (does nothing yet, no tests yet)
and a 'dev' target, which runs linters, tests, and finally builds.

scripts/version-at-commit.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+set -e -o pipefail
+commit="$1"
+versionglob="v[0-9].[0-9]*.[0-9]*"
+devsuffix=".dev"
+if [ -z "$commit" ]; then
+  commit=`git log -n1 --first-parent "--format=format:%h"`
+fi
+
+# automatically assign version
+#
+# handles the following cases:
+#
+# 0. no tags on the repository. Print "dev".
+#
+# 1. no local modifications and commit is directly tagged. Print tag.
+#
+# 2. no local modifications and commit is not tagged. Take greatest version tag in repo X.Y.Z and assign X.Y.(Z+1). Print that + $devsuffix + $timestamp.
+#
+# 3. local modifications. Print "dev".
+
+tags=$(git tag)
+if [[ -z "$tags" ]]; then
+  echo "dev"
+elif `git diff --quiet 2>/dev/null`; then
+  tagged=$(git tag --points-at "$commit")
+  if [[ -n "$tagged" ]] ; then
+    echo $tagged
+  else
+    nearest_tag=$(git describe --tags --abbrev=0 --match "$versionglob" "$commit")
+    v=$(echo $nearest_tag | perl -pe 's/(\d+)$/$1+1/e')
+    isodate=$(TZ=UTC git log -n1 --format=%cd --date=iso "$commit")
+    ts=$(TZ=UTC date --date="$isodate" "+%Y%m%d%H%M%S")
+    echo "${v}${devsuffix}${ts}"
+  fi
+else
+  echo "dev"
+fi