* ltmain.in: create on-demand executable as $progdir/$$-$program,

* ltmain.in: create on-demand executable as $progdir/$$-$program,
and rename it to $progdir/$program only when it's finished, to
avoid race conditions.  We can still get the program linked
multiple times, if multiple instances are started simultaneously
and the program does not exist, but they are very unlikely to
interfere with each other.  There's still a possibility that one
process removes the $program another has just created, and doesn't
replace it before the other tries to run it, in a system whose
`mv' is not atomic, so it will $rm then $mv, but so what? :-)
Locking files have been avoided to prevent dead-locks in case they
are left over after a reboot or crash.