Makefile: Document sed command and parameterise UCD version

Makefile

@@ -1,6 +1,10 @@
+UCD:=16.0.0
+
+# the sed command removes entries in the table that map to themselves, which
+# makes the data tables a bit smaller.
 tables:
-	yeslogic-ucd-generate case-folding-simple --rust-match ../ucd-generate/ucd-16.0.0/ > src/case_folding_simple.rs
-	yeslogic-ucd-generate case-mapping ../ucd-generate/ucd-16.0.0/ > src/tables.rs
+	yeslogic-ucd-generate case-folding-simple --rust-match ../ucd-generate/ucd-$(UCD)/ > src/case_folding_simple.rs
+	yeslogic-ucd-generate case-mapping ../ucd-generate/ucd-$(UCD)/ > src/tables.rs
 	cargo fmt
 	sed -i.bak -E '/\(([0-9]+), &\[\1\]\)/d' src/tables.rs
 	rm src/tables.rs.bak