samp and friends from char* to const char*

--HG--
branch : trunk
diff --git a/jinja2/_speedups.c b/jinja2/_speedups.c
index 30bea54..6aad8ec 100644
--- a/jinja2/_speedups.c
+++ b/jinja2/_speedups.c
@@ -11,7 +11,7 @@
 #include <Python.h>
 
 
-static char *samp = "&amp;", *slt = "&lt;", *sgt = "&gt;", *sqt = "&quot;";
+static const char *samp = "&amp;", *slt = "&lt;", *sgt = "&gt;", *sqt = "&quot;";
 static Py_UNICODE *amp, *lt, *gt, *qt;
 static PyObject* markup;