autotools: proper fix for generated Makevars and distcheck

Reverts 21da4a5454.

It is needed to generate Makevars in-tree even when building out-of-tree because of
how Autotools work. However, distcheck doesn't properly remove the Makevars file in
the generated distdir and makes po/ read only, preventing the build system from
generating an up-to-date version of Makevars. This commit adds the required hooks
needed to fix this behavior.

Makefile.am

@@ -431,5 +431,14 @@ pkgbuild::
 clean-local:
 	rm -rf benchmark coverage
 
+# Makevars is generated in top source directory, even for out-of-tree
 distclean-local:
-	rm -f po/Makevars
+	rm -f $(top_srcdir)/po/Makevars
+
+# we need this to remove Makevars from dist
+dist-hook:
+	rm -f $(distdir)/po/Makevars
+
+# in dist tree po is read only, and we need to generate Makevars in there
+distcheck-hook:
+	chmod +w $(distdir)/po

configure.ac

@@ -247,14 +247,14 @@ AM_GNU_GETTEXT_VERSION([0.17])
 m4_ifdef([AC_GNU_GETTEXT], [
 AC_GNU_GETTEXT([external])
 po_makefile_in=po/Makefile.in
-po_makevars=po/Makevars
+po_makevars="${srcdir}/po/Makevars:po/Makevars.in"
 have_po="yes"
 ],
 [
 m4_ifdef([AM_GNU_GETTEXT], [
 AM_GNU_GETTEXT([external])
 po_makefile_in=po/Makefile.in
-po_makevars=po/Makevars
+po_makevars="${srcdir}/po/Makevars:po/Makevars.in"
 have_po="yes"
 ],
 [