NarUtil: fix whitespace
NarUtil: log executed commands verbatim

This is nice to see what NAR is actually doing at each step.

We use the "+" prefix notation, same as "set -x" does.
MacOSX: Include Java header's subdirectory darwin/, too

This is needed with Java 7 on MacOSX because jni_md.h lives in the
darwin/ subdirectory now (in contrast to earlier Java versions, where it
was in the same directory as jni.h).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>