Report errors in preprocessor expressions by default

Until now SWIG quietly ignored such errors unless -Wextra (or -Wall
which implies -Wextra) was passed, but this is unhelpful as it hides
problems.  To illustrate this point, enabling this warning by
default revealled a typo in the preproc_defined.i testcase in
SWIG's own testsuite.

If you really don't want to see this warning, you can suppress it
with command line option -w202 or by using this in your interface
file:

%warnfilter(SWIGWARN_PP_EVALUATION);

Both will work with older versions of SWIG too.

Fixes #1465
Fixes #2389

Examples/test-suite/errors/cpp_pp_expressions_bad.i

@@ -1,5 +1,5 @@
 %module xxx
-/* Note: needs -Wextra to see these warnings */
+
 
 /* Spaceship operator doesn't seem to be allowed in preprocessor expressions. */
 #if (4 <=> 2) < 0

Examples/test-suite/errors/pp_expressions_bad.i

@@ -1,5 +1,5 @@
 %module xxx
-/* Note: needs -Wextra to see these warnings */
+
 
 /* Divide by zero */
 #define ZERO 0

Examples/test-suite/preproc_defined.i

@@ -86,7 +86,7 @@ struct Defined {
 void defined_not(TYPE);
 #endif
 
-#if !( defined(AAA) \
+#if !( defined(AAA) &&\
  defined(BBB) \\
 && defined(CCC) )
 void bumpf_not(TYPE);