Turn on more compiler warnings

* configure.ac: Enable gcc -Wwrite-strings.
diff --git a/configure.ac b/configure.ac
index 3a392aa..483694c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -172,6 +172,7 @@
 fi
 
 gl_WARN_ADD([-Wall])
+gl_WARN_ADD([-Wwrite-strings])
 if test "x$opsys" = "xsunos4"
 then
 	# SunOS 4.x header files don't declare int functions.