out-of-source Configure succeeds when it fails

[tonycoz]

Based on Perl/perl5#16725

Perl's Configure has a mechanism to do out of source builds (-Dmksymlinks) but if the user tries to do such a build without that option, Configure produces an error message:

Run make depend now? [y]  
make: *** No rule to make target 'Makefile.SH', needed by 'Makefile'.  Stop.
...

but $? reports success:

tony@mars:.../git/outoftree$ echo $?
0

From going over the metaconfig source this could be fixed in dist/U/Finish.U, but to be friendlier it should also produce a message mentioning -Dmksymlinks, and I'm not sure if that belongs in dist/U/Finish.U

Ideally we'd detect an out-of-source build earlier and either:

a) report an error earlier, and exit 1 so calling tools do the right thing

b) automatically turn on -Dmksymlinks (though this can have unfriendly effects too)