gitinfo.txt on Unix

Hide the scary perl command during 'make all' feedback in src/.
I'm not a shell programmer but this works fine for me when skipping
NHgithook::NHversioning because dat/gitinfo.txt is already present,
when creating dat/gitinfo.txt successfully, and when creation fails
because perl can't find NHgithook.pm.  It hasn't been tested when
perl itself is absent.

sys/unix/gitinfo.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+# NetHack 3.6  gitinfo.sh	$NHDT-Date: 1520201830 2018/03/04 22:17:10 $  $NHDT-Branch: NetHack-3.6.0 $:$NHDT-Revision: 1.0 $
+
+# if dat/gitinfo.txt doesn't exist, try to make one; called from Makefile.src
+
+#
+# gitinfo.txt is used during development to augment the version number
+# (for nethack's 'v' command) with more specific information.  That is not
+# necessary when building a released version and it is perfectly OK for
+# this script to be skipped or to run but fail to generate dat/gitinfo.txt.
+#
+
+# try to figure out where we are: top, one level down (expected), or sys/unix
+prefix=.
+if [ -f ../sys/unix/gitinfo.sh ]; then prefix=..; fi
+if [ -f ../../sys/unix/gitinfo.sh ]; then prefix=../..; fi
+
+# try to run a perl script which is part of nethack's git repository
+if [[ ! -f $prefix/dat/gitinfo.txt ]]; then
+  ( cd $prefix; \
+    perl -IDEVEL/hooksdir -MNHgithook -e '&NHgithook::nhversioning' \
+      2> /dev/null ) \
+  || true
+fi
+exit 0