doc: fix symlink failure when installing libcaca twice.

doc/Makefile.am

@@ -44,7 +44,7 @@ clean-local:
 install-data-local:
 if BUILD_DOCUMENTATION
 	mkdir -p $(DESTDIR)$(datadir)/doc
-	ln -s libcaca-dev $(DESTDIR)$(datadir)/doc/libcucul-dev
+	ln -sf libcaca-dev $(DESTDIR)$(datadir)/doc/libcucul-dev
 	$(mkinstalldirs) $(DESTDIR)$(datadir)/doc/libcaca-dev/html
 	cp `find html -name '*.html' -o -name '*.gif' -o -name '*.png'` \
 	  $(DESTDIR)$(datadir)/doc/libcaca-dev/html
@@ -55,7 +55,7 @@ if BUILD_DOCUMENTATION
 	done
 	$(mkinstalldirs) $(DESTDIR)$(mandir)/man1
 	rm -f $(DESTDIR)$(mandir)/man1/cacademo.1
-	ln -s cacafire.1 $(DESTDIR)$(mandir)/man1/cacademo.1
+	ln -sf cacafire.1 $(DESTDIR)$(mandir)/man1/cacademo.1
 endif
 
 uninstall-local: