Calculate VERSION and RELEASE dynamically from git

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

scripts/get-version.sh

@@ -0,0 +1,77 @@
+#!/bin/sh
+# Copyright (c) Contributors to the Apptainer project, established as
+#   Apptainer a Series of LF Projects LLC.
+#   For website terms of use, trademark policy, privacy policy and other
+#   project policies see https://lfprojects.org/policies
+
+# optionally take commit-ish as argument for debugging purposes
+head=$1
+
+gitrepo=true
+toplevel="$(git rev-parse --show-toplevel 2> /dev/null)"
+
+if test -z "${toplevel}" ; then
+	# failed to get toplevel directory, we are not inside a git
+	# repo?
+	gitrepo=false
+	toplevel=$PWD
+fi
+
+if test -f "${toplevel}/VERSION" ; then
+	cat "${toplevel}/VERSION"
+	exit 0
+fi
+
+# no VERSION file
+
+if ! ${gitrepo} ; then
+	echo "E: Not inside a git repository and no VERSION file found. Abort." 1>&2
+	exit 1
+fi
+
+# gitdesc extracts the closest version tag, removing the leading "v" and
+# returning the rest
+gitdesc() {
+	git describe --match='v[0-9]*' --always --candidates=1 "$@" 2>/dev/null |
+	sed -e 's,^v,,'
+}
+
+closest_tag=$(gitdesc --abbrev=0 ${head})
+current_commit=$(gitdesc ${head})
+if test -n "${head}" ; then
+	# cannot pass --dirty when using commit-ish
+	dirty_info=${current_commit}
+else
+	dirty_info=$(gitdesc --dirty)
+fi
+
+# First figure out if the current commit matches a tag:
+#
+# v3.4.1
+# v3.4.1-rc.1
+
+if [ "${closest_tag}" = "${current_commit}" ] ; then
+	# We are on a tag, leave it alone, maybe including the dirty
+	# mark.
+	#
+	# This works even if we ever use a tag like v3.4.1+pl1 (which we
+	# will hopefully never have).
+	echo "${dirty_info}"
+	exit 0
+fi
+
+# We have something which doesn't exactly match a tag:
+#
+# v3.4.1-315-g21e6d6765
+# v3.4.1-rc.1-315-g21e6d6765
+#
+# Use the -315-g21e6d6765 part as metadata, using + to indicate that
+# this is 3.4.1 with 315 additional changes. Keep the g21e6d6765 part to
+# have a record of the corresponding git commit hash.
+#
+# Transform the above into:
+#
+# v3.4.1+315-g21e6d6765
+# v3.4.1-rc.1+315-g21e6d6765
+
+echo "${dirty_info}" | sed -e "s,^\(${closest_tag}\)-,\1+,"