Cygwin: Have cygmagic not create output if an error occurs

Improve the 'cygmagic' script, so it doesn't create the output file if
an error occurs, even in one of the backtick-enclosed pipelines it runs.

winsup/cygwin/cygmagic

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # cygmagic - Generate "magic numbers" from a structure.
 #
 # This file is part of Cygwin.
@@ -7,14 +7,22 @@
 # Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
 # details.
 
+set -e
+shopt -s -o pipefail
+shopt -s inherit_errexit
+
 file_magic=$1; shift
 gcc=$1; shift
 file=$1; shift
+
+tmpfile=/tmp/$$.magic
 trap "rm -f /tmp/$$.magic" 0 1 2 15
-cat <<EOF > $file_magic
+
+cat <<EOF > $tmpfile
 /* autogenerated - do not edit */
 #include "$file"
 EOF
+
 sumit() {
     cksum $*
 }
@@ -28,5 +36,8 @@ while [ -n "$1" ]; do
     [ "$curr" != "$sum" ] && echo "*** WARNING WARNING WARNING WARNING WARNING ***
 *** $file: magic number for $define changed old $curr != new $sum
 *** WARNING WARNING WARNING WARNING WARNING ***" 1>&2
-done >> $file_magic
+done >> $tmpfile
+
+mv $tmpfile $file_magic
+
 exit 0