[libusb] Fix for linux builds <2.6.23

[Matthias B. <mat...@la...>]

Hi Everyone,

with the introduction of O_CLOEXEC the linux builds before 2.6.23 do not 
compile any more.
This could be fixed in the configure or before the inclusion of the fcntl.h.
I'm not the one adapting make processes, so I fixed this issue before 
the inclusion of the header file.
I created a pull request (#286) if you want this to merge.

Regards,
Matthias