make --disable-libgcrypt mandatory if (a recent enough) libgcrypt was not found

Product works for PCI as well, whereas model is only set for USB.

This implements device discovery with libudev, which is part of the udev
source package (and hence available in any recent Linux distribution).
Both cold and hot plugging (w.r.t. VLC) are implemented.

TODO:
 * V4L1 devices (currently V4L2 is assumed)
 * other subsystems (linux-dvb -> DVB, block -> discs)
 * item removal on unplug
 * better item name and category for non-USB devices

Original php-code made by Toni Korhonen, and I made lua script based
on that example.

macosx/framework: Fix the configure ARCHS autodetection script so that we are shell independant (setenv vs export), and force to build only the selected ARCHS.

We still need the configure checks.
Only the HAL device probe plugin is gone.

It was ever dead code within VLC. On top of that, HAL is now officially
deprecated in favor of libudev (and higher level D-Bus middlewares).

This should fix a rare crash:
LibVLC fatal error destroying mutex (16) in thread 17390 at ../../src/misc/pthread.c:188 in vlc_mutex_destroy
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4[0xb7fff933]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4[0xb7fff9c8]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4(vlc_mutex_destroy+0x3f)[0xb7fffa39]
/home/remi/videolan/vlc/build/modules/access/rtp/.libs/librtp_plugin.so[0xb5563a41]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4(__module_unneed+0x29)[0xb7fe414f]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4[0xb7f9984e]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4[0xb7fa69fb]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4[0xb7fa6c0c]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4[0xb7faeef2]
/home/remi/videolan/vlc/build/src/.libs/libvlccore.so.4[0xb7feb16e]
/lib/i686/cmov/libpthread.so.0[0xb7f324b5]
/lib/i686/cmov/libc.so.6(clone+0x5e)[0xb7eaea5e]

Signed-off-by: Laurent Aimar <fenrir@videolan.org>