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.
Merge pull request #7 from jussy/master

Fix compilation on Solaris 11