Build fails on macOS: `xpc.h: fatal error: os/object.h: No such file or directory`

[barracuda156]

The build fails on non-exiting headers:

In file included from /opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_devel_libxpc/libxpc/work/libxpc-c70dc7d15ae429ae62522e7566aa25772a09b0aa/xpc_connection.c:29:
/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_devel_libxpc/libxpc/work/libxpc-c70dc7d15ae429ae62522e7566aa25772a09b0aa/xpc/xpc.h:7:10: fatal error: os/object.h: No such file or directory
    7 | #include <os/object.h>
      |          ^~~~~~~~~~~~~
compilation terminated.

/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_devel_libxpc/libxpc/work/libxpc-c70dc7d15ae429ae62522e7566aa25772a09b0aa/xpc_misc.c:31:10: fatal error: machine/atomic.h: No such file or directory
   31 | #include <machine/atomic.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.

@jceel os/object.h does not exist before 10.8: https://github.com/phracker/MacOSX-SDKs/tree/master/MacOSX10.7.sdk/usr/include
So it should be used conditionally.

I have no idea about machine/atomic.h, maybe it is some kernel private header. I cannot find it in the SDK. There are stdatomic.h and libkern/OSAtomic.h.

Also, -fblocks is a clangism, it is not supported with gcc.