Makefile: Fix broken build

make -C mybuilddir no longer works (regression caused by commit)
388d4758.

PWD is the directory of the caller (not mybuilddir),
so BUILD_DIR is set to the wrong value.

GNU make sets CURDIR to the correct value.
Use this macro instead of PWD.

Cc: Lluís Vilanova <vilanova@ac.upc.edu>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>