build: Add rules for building text version of manual page.

Makefile.am

@@ -100,6 +100,14 @@ goaljobs-reference.1: goaljobs-reference.pod
 	$(POD2MAN) -c "Goaljobs" --release $(PACKAGE)-$(VERSION) $< > $@-t
 	mv $@-t $@
 
+goaljobs.1.txt: goaljobs.pod
+	pod2text $< > $@-t
+	mv $@-t $@
+
+goaljobs-reference.1.txt: goaljobs-reference.pod
+	pod2text $< > $@-t
+	mv $@-t $@
+
 endif
 
 if HAVE_OCAMLDOC