Compile .el files at O(1) instead of O(n)

The older code was running Emacs separately for each .el file. Change
the code so that all .el files are passed at once.

This improves build time by x5:

* before: 1.272 sec
* after:  0.257 sec

Also simplify the rule declaration.

Author: Hi-Angel

Makefile

@@ -35,7 +35,7 @@ PKG_TAR = purescript-mode-$(VERSION).tar
 
 %.elc: %.el
 	@$(BATCH) \
-	   --eval "(setq byte-compile-error-on-warn t)" -f batch-byte-compile $<
+	   --eval "(setq byte-compile-error-on-warn t)" -f batch-byte-compile $(ELFILES)
 
 .PHONY: all compile info clean test elpa package