The current header files of XrdPosix do not properly work when used in a C context. Compiling this reproducer:

#include <stdio.h>
#include <XrdPosix/XrdPosix.hh>
int main(){
}

With clang this fails because the code tries to include cstdio instead of stdio.h in C context. After fixing this, it works with clang but not with gcc. There are 2 more issues:

• with gcc 11 and newer the code fails to compile when dirent.h is included. This can be fixed by re-arranging the code properly
• Some of the types defined in XrdPosixExtern.hh conflict with the system defaults, e.g. long long should be __off64_t or int functions are actually size_t. Newer compiler versions complain about this. The patch adapts the function definitions accordingly.