[XrdCl] Switch the default poller implementation to libevent as the built-in works unreliably on MacOS and Solaris
Fix compilation on Solaris 11

Solaris 11 defines a different DIR structure by default than Solaris 10. The __USE_LEGACY_PROTOTYPES__ variable, if defined, makes Solaris 11 use the Solaris 10 structure. So we define it in XrdSysPlatform.hh.