more descriptive "make" message

[Bret Victor <bret@...>]

Sorry about noticing this at rc4, but I was out of the Lua loop for a
while.

When I type "make", I get:

 Please choose a platform: ansi bsd generic linux macosx mingw posix solaris

This doesn't tell me what to do next.  Perhaps a more informative
message would be something like:

 Please type "make [PLATFORM]", where [PLATFORM] is one of: ansi bsd generic linux macosx mingw posix solaris

-Bret