README: Add OS X specific notes about the lack of libtoolize
Core: defensive programming

Defensively set return-by-reference value to -1 in error condition
Core: fix 64-bits warning on OS X

suppress 64 bit truncation warning on OS X due to tv_sec being a __darwin_time_t which is a long, not an int
On gcc 4 and later, suppress the -Wcast-align warning in libusb.h