w32: Fix gnupg_unsetenv.

* common/sysutils.c (gnupg_unsetenv): Don't use nonstandard extension
of "NAME", but "NAME=".

--

Microsoft implementation of putenv works to remove an environment
variable by "NAME=".

POSIX doesn't say that putenv with "NAME=" has same effect.  GNU
implementation doesn't support this way for removal of environment
variable.

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

common/sysutils.c

@@ -1192,19 +1192,28 @@ gnupg_unsetenv (const char *name)
 #else /*!HAVE_UNSETENV*/
   {
     char *buf;
+    int r;
 
     if (!name)
       {
         gpg_err_set_errno (EINVAL);
         return -1;
       }
-    buf = xtrystrdup (name);
+    buf = strconcat (name, "=", NULL);
     if (!buf)
       return -1;
+
+    r = putenv (buf);
+# ifdef HAVE_W32_SYSTEM
+    /* For Microsoft implementation, we can free the memory in this
+       use case.  */
+    xfree (buf);
+# else
 #  if __GNUC__
 #   warning no unsetenv - trying putenv but leaking memory.
 #  endif
-    return putenv (buf);
+# endif
+    return r;
   }
 #endif /*!HAVE_UNSETENV*/
 }