[PHP7] Make default C++ extension .cxx

This is SWIG's default for every language except PHP, and now is a
good time to make this change so that once we drop PHP5 support, we'll
have the same default everywhere.  It's easy to override the default
with -cppext cxx to get the PHP5 behaviour.

Source/Modules/main.cxx

@@ -64,7 +64,7 @@ static const char *usage1 = (const char *) "\
      -copyctor       - Automatically generate copy constructors wherever possible\n\
      -cpperraswarn   - Treat the preprocessor #error statement as #warning (default)\n\
      -cppext <ext>   - Change file extension of generated C++ files to <ext>\n\
-                       (default is cxx, except for PHP which uses cpp)\n\
+                       (default is cxx, except for PHP5 which uses cpp)\n\
      -copyright      - Display copyright notices\n\
      -debug-classes  - Display information about the classes found in the interface\n\
      -debug-module <n>- Display module parse tree at stages 1-4, <n> is a csv list of stages\n\