port commit-msg to Gawk 3.0.4 (1999)

* build-aux/git-hooks/commit-msg (cent_sign_utf8_format, cent_sign)
(print_at_sign, at_sign): New vars.  Use them to avoid problems
Eli Zaretskii encountered with Gawk 3.0.4 (1999) on MSYS.  See:
http://lists.gnu.org/archive/html/emacs-devel/2015-04/msg00566.html

build-aux/git-hooks/commit-msg

@@ -29,34 +29,36 @@ fi
 
 # Use a UTF-8 locale if available, so that the UTF-8 check works.
 # Use U+00A2 CENT SIGN to test whether the locale works.
-cent_sign_utf8_octal='\302\242'
-at_sign=`
-  printf "${cent_sign_utf8_octal}@" |
-  $awk '{print substr($0, 2)}' 2>/dev/null
-`
+cent_sign_utf8_format='\302\242\n'
+cent_sign=`printf "$cent_sign_utf8_format"`
+print_at_sign='{print substr("'$cent_sign'@", 2)}'
+at_sign=`$awk "$print_at_sign" 2>/dev/null`
 if test "$at_sign" != @; then
-  at_sign=`
-    printf "${cent_sign_utf8_octal}@" |
-    LC_ALL=en_US.UTF-8 $awk '{print substr($0, 2)}' 2>/dev/null
-  `
+  at_sign=`LC_ALL=en_US.UTF-8 $awk "$print_at_sign" 2>/dev/null`
   if test "$at_sign" = @; then
     LC_ALL=en_US.UTF-8; export LC_ALL
   fi
 fi
 
 # Check the log entry.
-exec $awk '
+exec $awk -v at_sign="$at_sign" -v cent_sign="$cent_sign" '
   BEGIN {
+    # These regular expressions assume traditional Unix unibyte behavior.
+    # They are needed for old or broken versions of awk, e.g.,
+    # mawk 1.3.3 (1996), Gawk 3.0.4 (1999).
+    space = "[ \f\n\r\t\v]"
+    non_space = "[^ \f\n\r\t\v]"
+    non_print = "[\1-\37\177]"
+
+    # Prefer POSIX regular expressions if available, as they do a
+    # better job of checking.  Similarly, prefer POSIX negated
+    # expressions if UTF-8 also works.
     if (" " ~ /[[:space:]]/) {
       space = "[[:space:]]"
-      non_space = "[^[:space:]]"
-      non_print = "[^[:print:]]"
-    } else {
-      # mawk 1.3.3 does not support POSIX bracket expressions.
-      # Approximate them as best we can.
-      space = "[ \f\n\r\t\v]"
-      non_space = "[^ \f\n\r\t\v]"
-      non_print = "[\1-\37\177]"
+      if (at_sign == "@" && cent_sign ~ /^[[:print:]]$/) {
+        non_space = "[^[:space:]]"
+        non_print = "[^[:print:]]"
+      }
     }
   }