abuild-clean: add option to make files writable before cleanup

Some projects might leave files which are not writable for the current
user. The cleanup process then fails and leaves files / directories
behind.

This can easily be fixed by making everything writable before removing
the files.

Add the option 'chmod-clean' which does just that.

abuild.in

@@ -77,6 +77,9 @@ want_check() {
 }
 
 default_cleanup_srcdir() {
+	if option_has options_has "chmod-clean" && test -d "$srcdir"; then
+		chmod -R +w "$srcdir"
+	fi
 	rm -rf "$srcdir"
 }