Work around _ABS macro on SunOS

See https://trac.sagemath.org/ticket/24619

diff -ru a/src/rpn.h b/src/rpn.h
--- a/src/rpn.h	2016-01-03 09:12:13.000000000 +0100
+++ b/src/rpn.h	2018-01-31 10:36:43.049921736 +0100
@@ -24,6 +24,10 @@
 #include <string>
 #include <ctype.h>
 
+/* SunOS defines this as macro */
+#undef _ABS
+
+
 #ifndef NO_NAMESPACE_GIAC
 namespace giac {
 #endif // ndef NO_NAMESPACE_GIAC
