fix the build on OS X

OS X doesn't have posix_fallocate() yet, so put
bf760d0241f0f078735e230b4bf6da4fc83368fe in
    #if defined(__APPLE__)

the cd fails with:
    /bin/sh: line 0: cd: include: No such file or directory
so give it a path relative to the top directory
ignore symbol files on OS X (only for debugging, breaks git-add -A)
libi3/root_atom_contents: handle data of arbitrary length

Handle data fetched from xcb_get_property_unchecked with arbitrary
length. This avoids having to rely on PATH_MAX macro where it is not
necessary.