Support building with the Perl XS bindings disabled

Since the Perl bindings require shared libraries, this is required on
platforms such as Cygwin where we do a static build.

substitute.mk

@@ -30,6 +30,7 @@
 	 -e "s^@xsltproc\@^$(xsltproc)^g" \
 	 -e "s^@sqlite_bin\@^$(sqlite_bin)^g" \
 	 -e "s^@version\@^$(VERSION)^g" \
+	 -e "s^@perlbindings\@^$(perlbindings)^g" \
 	 -e "s^@testPath\@^$(coreutils):$$(dirname $$(type -p expr))^g" \
 	 < $< > $@ || rm $@
 	if test -x $<; then chmod +x $@; fi