Improve error message when preprocessor expressions result in a floating point constant

Instead of a syntax error, the error is now:
  Warning 202: Error: 'Floating point constant in preprocessor expression'

Examples/test-suite/errors/pp_expressions_bad.i

@@ -1,5 +1,7 @@
 %module xxx
+/* Note: needs -Wextra to see these warnings */
 
+/* Divide by zero */
 #define ZERO 0
 
 #if 1%ZERO
@@ -11,3 +13,14 @@
 #endif
 #if 2/(55-55)
 #endif
+
+/* Floating point */
+#if 1.2
+#endif
+
+#if 2e3
+#endif
+
+#if 0
+#elif 8.8
+#endif