new patch to Makefile for building as stand alone.

I've been thinking about the patch I submitted and have concluded that it could have been much better.

Now that the patch has been applied, if someone wants to build and install the exfat module for a kernel other than the one currently running,
(s)he has to provide values for both KDIR and MDIR when calling make.
Clearly that is not good.
That can, however, be trivially fixed by changing the assignment to MDIR as follows:

MDIR   := $(shell dirname ${KDIR})

In addition, however, in my experience of building external modules a user can set KDIR to any path to the kernel source.
For example, on my system the source for the currently-running kernel is in $HOME/kernel/linux-$(uname-r)
and it should be possible to use that as a value for KDIR instead of /lib/modules/$(uname -r)/build.
The patch below implements this idea as well as fixing the KDIR+MDIR badness I describe above.

Trivially, the patch also aligns the assignment to PWD with the earlier assignments.

Signed-off-by: Chris Clayton <chris2553@googlemail.com>