[PATCH] better allow builds of extras programs under udev

Here is an improved version of the patch that enables builds of the extras
progams for the targets all, clean, install, and uninstall, and passes
down the "prefix" for use by install and uninstall.

This patch enables building of the "extras" programs using the same build
environment as udev (i.e. build with udev's versions of klibc and
sysfsutils).

For example, build scsi_id and udev via:

	make EXTRAS=extras/scsi_id

Build scsi_id and udev with klibc via:

	make KLIBC=true EXTRAS=extras/scsi_id