Add a variant of `locale_charset' that returns its result based solely on
information from the environment.  See
http://lists.gnu.org/archive/html/guile-devel/2011-11/msg00040.html for the
rationale.

diff --git a/lib/localcharset.h b/lib/localcharset.h
index 8907ccd..43e976f 100644
--- a/lib/localcharset.h
+++ b/lib/localcharset.h
@@ -32,6 +32,12 @@ extern "C" {
    name.  */
 extern const char * locale_charset (void);
 
+/* Same as above, but only look at environment variables, avoiding calls to
+   `setlocale', `nl_langinfo', etc.  See
+   <http://lists.gnu.org/archive/html/guile-devel/2011-11/msg00040.html> for
+   the rationale.  */
+extern const char * environ_locale_charset (void);
+
 
 #ifdef __cplusplus
 }
