Add `LIBUSB_ENABLE_UDEV` option

[ririyeye]

there is no udev support in some embedded system , they need a option work without libudev and if we compile sys/socket.h we need to add -D_GNU_SOURCE according to stackoverflow

[Youw]

if we compile sys/socket.h we need to add -D_GNU_SOURCE according to stackoverflow

Is it causing a build issue? Which system/compiler?

In any case - lets not introduce two independent changes in one PR.

[Youw]

nice improvement overall, thanks

[ririyeye]

if we compile sys/socket.h we need to add -D_GNU_SOURCE according to stackoverflow

Is it causing a build issue? Which system/compiler?

In any case - lets not introduce two independent changes in one PR.

in linux-gcc

it may cause build err to linux_netlink.c

i figure out in autoconf , libusb will add _GNU_SOURCE in config.h file , and it add -DHAVE_CONFIG_H flags to include config.h so it works

[Youw]

i figure out in autoconf , libusb will add _GNU_SOURCE in config.h file , and it add -DHAVE_CONFIG_H flags to include config.h so it works

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