Makefile: Simplify the conditional message of the all target.

Makefile: Simplify the conditional message of the all target.

We wamt a simple "make" to call the 'all' target and then print a
message when done, but we don't want "make install" which depends on
that same 'all' target to print the message.

We previously did this with a separate 'all-without-message' target,
which was inelegant because it caused all users of the target to
carefully depend on 'all-without-message' rather than 'all'.

Instead, we now use a single 'all' target but with a Makefile
conditional that examines the MAKECMDGOALS variable to determine
whether to print the message.