Ensure correct version in dist tarball

That adds an additional dist hook that verifies that the PACKAGE_VERSION
that configure records is the same as the _current_ version reported by
`git describe`.  PACKAGE_VERSION is only updated when _autoconf_ is
run, so if you've made several commits, the version number will be out
of date.  You have to explicitly re-run autogen.sh to regenerate the
correct package version.

h/t https://stackoverflow.com/a/46222039

Makefile.am

@@ -4,13 +4,33 @@
 # Please see the COPYING file in this distribution for license details.
 # ------------------------------------------------------------------------------
 
+#-------------------------------------------------------------------------------
+# Dist hooks
+
+dist-hook: dist-check-git-version dist-stamps
+
 #-------------------------------------------------------------------------------
 # .commit-stamp and .version-stamp
 
-# These should only appear in distribution tarballs.
-dist-hook:
-	echo $(GIT_SHA1) > $(distdir)/.commit-stamp
-	echo $(VERSION) > $(distdir)/.version-stamp
+dist-check-git-version:
+	: # Verify that the version that configure.ac recorded matches the
+	: # current version from git tags.
+	@if test -n $(top_srcdir)/.version-stamp; then \
+	    git_ver=`$(top_srcdir)/build-aux/calculate version`; \
+	    if test "x$${git_ver}" != "x$(PACKAGE_VERSION)"; then \
+	        echo "ERROR: PACKAGE_VERSION and 'git describe' version do not match:"; \
+	        echo "     current 'git describe' version: $${git_ver}"; \
+	        echo "     current PACKAGE_VERSION:        $(PACKAGE_VERSION)"; \
+	        echo "Update PACKAGE_VERSION by running $(top_srcdir)/autogen.sh."; \
+	        rm -rf "$(top_srcdir)/autom4te.cache"; \
+	        exit 1; \
+	    fi \
+	fi
+
+dist-stamps:
+	@: # Generate version stamps for dist tarball.
+	@echo $(GIT_SHA1) > $(distdir)/.commit-stamp
+	@echo $(VERSION) > $(distdir)/.version-stamp
 
 #-------------------------------------------------------------------------------
 # Preliminaries

autogen.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+# ------------------------------------------------------------------------------
+# Copyright © 2020, libcork authors.
+# Please see the COPYING file in this distribution for license details.
+# ------------------------------------------------------------------------------
+
+set -e
+
+cd "$(dirname "$0")"
+
+AUTORECONF=${AUTORECONF:-autoreconf}
+ACLOCAL=${ACLOCAL:-aclocal}
+AUTOCONF=${AUTOCONF:-autoconf}
+AUTOHEADER=${AUTOHEADER:-autoheader}
+AUTOMAKE=${AUTOMAKE:-automake}
+
+"${AUTORECONF}" --verbose --install --force -I m4