libcamera: Make libudev optional

libcamera depends on libudev for device enumeration. It is however
useful to allow building documentation without requiring the dependency
to be installed. Make the libudev dependency optional and compile the
udev-based device enumerator out when libudev is not present.

Note that while libcamera will compile without libudev, it will not be
able to enumerate devices. A sysfs-based device enumerator is planned as
a fallback but not implemented yet.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/camera_manager.cpp

@@ -80,7 +80,7 @@ int CameraManager::start()
 		return -EBUSY;
 
 	enumerator_ = DeviceEnumerator::create();
-	if (enumerator_->enumerate())
+	if (!enumerator_ || enumerator_->enumerate())
 		return -ENODEV;
 
 	/*