Add comment about TMPDIR.
#
################################################################################
+# Directory used by GHC (and possibly other tools) for storing
+# temporary files. If your /tmp isn't big enough, either override
+# this in build.mk or set your environment variable "TMPDIR" to point
+# to somewhere with more space. (TMPDIR=. is a good choice).
+
TMPDIR = /tmp
# FPTOOLS_TOP: the top of the fptools hierarchy, absolute path.