Rename build-time option TMPDIR to EXIM_TMPDIR

Build-time option TMPDIR included in Makefile clashes with environment
variable of the same name. This breaks tools that make use of that
variable, such as distcc.

The following example demonstrates what's going on:

$ cat Makefile
TMPDIR="/tmp"
all:
	env
$ export TMPDIR=test
$ make | grep ^TMPDIR
TMPDIR="/tmp"

distcc error (note the extra quotes):
ERROR: can't use TMPDIR ""/tmp"": No such file or directory