abuild: add postcheck for required license file

Some licenses require distributing a copy of the license file[1], this
adds a postcheck to make sure that packages of software with one of
these licenses is meeting the requirement.

1. https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package#license

abuild.in

@@ -923,6 +923,19 @@ postcheck() {
 			e=1
 		fi
 	fi
+
+	# test for required license file
+	# https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package#license
+	if [ "$name" = "$pkgname" ]; then
+		local i; for i in $license; do
+			list_has "$i" AND OR WITH && continue
+			if list_has "$i" MIT ISC \
+					&& [ ! -f "$dir/usr/share/licenses/$pkgname/LICENSE" ]; then
+				error "Requires license but none found at /usr/share/licenses/$pkgname/LICENSE"
+				e=1
+			fi
+		done
+	fi
 	return $e
 }