Add Makefile 'default' target

The command `make` runs the first Makefile target.

Having the first target be called 'default' makes it obvious what is
going on.

I often specify no deps for the default target so running `make' prints:

    Nothing to be done for 'default'.

Author: ingydotnet

Makefile

@@ -67,6 +67,9 @@ GLJ-BINS=$(foreach platform,$(GO-PLATFORMS) \
 
 GO-CMD := go$(GO-VERSION)
 
+#-------------------------------------------------------------------------------
+default: all
+
 all: gocmd stdlib-targets generate aot glj-imports glj-bins
 
 gocmd: