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

Doc/Manual/SWIG.html

@@ -219,7 +219,7 @@ General Options
      -Wall           - Remove all warning suppression, also implies -Wextra
      -Wallkw         - Enable keyword warnings for all the supported languages
      -Werror         - Treat warnings as errors
-     -Wextra         - Adds the following additional warnings: 202,309,403,405,512,321,322
+     -Wextra         - Adds the following additional warnings: 309,403,405,512,321,322
      -w<list>        - Suppress/add warning messages, eg -w401,+321 - see Warnings.html
      -xmlout <file>  - Write XML version of the parse tree to <file> after normal processing
 </pre></div>

Doc/Manual/Warnings.html

@@ -164,6 +164,15 @@ to provide additional diagnostics.  These warnings can be turned on using the
 </pre>
 </div>
 
+<p>
+Preprocessor warning 202 ("Could not evaluate expression <em>expr</em>.") was
+formally off by default and enabled by <tt>-Wextra</tt>, but since SWIG 4.1.0
+this warning is on by default because suppressing it tends to hide genuine
+problems.  If you really don't want to see it, you can suppress it with
+<tt>-w202</tt> or using <tt>%warnfilter</tt> as described below. Both will work
+with older versions of SWIG too.
+</p>
+
 <p>
 To selectively turn on extra warning messages, you can use the directives and options in the
 previous section--simply add a "+" to all warning numbers.  For example: