Avoid GLib stdio macro wrappers when processing headers.
On unix, unless G_STDIO_NO_WRAP_ON_UNIX is defined, GLib defines part of its public API through macro definitions that map directly to system functions. This makes them invisible to g-ir-scanner.
Define G_STDIO_NO_WRAP_ON_UNIX when processing GLib headers. Fixes issue #170.
This extends gir xml with function definitions for: