Fix build breakage on Unix.

Fix build breakage on Unix.

Occurred as a side effect of commit 198bca233, in which I wrote a Perl
loop of the form 'foreach $srcdir (@srcdirs)' inside which I modified
$srcdir - forgetting the Perl gotcha that if you do that, $srcdir
temporarily aliases the actual array element, so you end up modifying
the array you iterated over. Hence, a set of transformations intended
to convert the source directory list into a special form for the nmake
batch-mode inference rule syntax in particular ended up back in
@srcdirs to be reflected in unrelated makefiles output later in the
run.