Make temp file in coding.test work on MS-Windows

[Eli Zaretskii]

I've built Guile 2.0.11 on another system, and found a problem in
coding.test: it assumes that /tmp exists, and creates the temporary
files there.  Here's the patch to make that more portable:

--- test-suite/tests/coding.test~0      2014-01-21 23:45:02.000000000 +0200
+++ test-suite/tests/coding.test        2014-08-09 13:16:46.416750000 +0300
@@ -20,7 +20,10 @@
   #:use-module (test-suite lib))
 
 (define (with-temp-file proc)
-  (let* ((name (string-copy "/tmp/coding-test.XXXXXX"))
+  (let* ((tmpdir (or (getenv "TMPDIR")
+                    (getenv "TEMP")
+                    "/tmp"))
+        (name (string-concatenate (list tmpdir "/coding-test.XXXXXX")))
          (port (mkstemp! name)))
     (let ((res (with-throw-handler
                 #t