Rename INCLUDE_MAKEFILES to MAKEFILES_DIR

[dbosk]

We should probably rename INCLUDE_MAKEFILES to MAKEFILES_DIR. However, this requires backwards compatibility:

INCLUDE_MAKEFILES?=.
MAKEFILES_DIR?=${INCLUDE_MAKEFILES}
include ${MAKEFILES_DIR}/portability.mk

Includes where this is done:

• doc.mk
• exam.mk
• haskell.mk
• noweb.mk
• pkg.mk
• portability.mk
• pub.mk
• results.mk
• subdir.mk
• tex.mk
• transform.mk