Core: defensive programming

Defensively set return-by-reference value to -1 in error condition

NB: The comments do not match the implementation.

Comments: "[return] the index of the configuration matching a specific
bConfigurationValue in the idx output parameter, or -1 if the config was
not found"

There is a code path where idx is never touched. Perhaps clients of the
function are careful to only read idx if the return value is success,
but also setting idx to -1 is much safer.
Core: correctly check usbi_read() returned value

For messages received on the hotplug pipe, the message was read via
usbi_read() (ssize_t) and compared against the size of the message
struct (size_t). usbi_read() returns -1 on an error condition, so some
systems can cast the ssize_t to size_t for the comparison, making it
equal to SIZE_MAX and causing the error check condition to incorrectly
evaluate to false.
README: Add OS X specific notes

Xcode 4.3 do not provide autotools anymore.
libusbx now provides a Xcode project to ease build Mac OS X.