Fixed: typo of method name that is called when c++ flag is true.

The following error occurs in Build.PL due to typo of method name.

```
$ perl Build.PL
Can't locate object method "have_cpluplus" via package "ExtUtils::CBuilder" at /path/to/lib/site_perl/5.16.3/Module/Build/Pluggable/XSUtil.pm line 78.
```