Makefile: shorten tip about Unicode embedding time

The estimate about this step taking "up to a minute" really depends on
the machine we're compiling on.  For example, on an old or underpowered
machine, it may take _longer_ than a minute.  So this patch changes the
wording accordingly and puts the reason for the delay at the beginning.
Merge pull request #2648 from sunaku/makefile-message-clarity

Makefile: shorten tip about Unicode embedding time