Use built-in $(CURDIR) instead of $(shell pwd).

Don't know if this will work on cygwin, so didn't update the documentation above the RUNDIR assignment line in mk/Makefile.

A comparison on Linux (Debian) with bash:

Makefile:
```make
SHELL := /bin/bash

$(info CURDIR            $(CURDIR))
$(info PWD               $(PWD))
$(info shell echo `pwd`  $(shell echo `pwd`))
$(info shell pwd         $(shell pwd))
$(error testing)
```
Output:
```
$ make -krj5 -C subdir all
CURDIR            /home/t/src/ideas/non-rec/subdir
PWD               /home/t/src/ideas/linkedrec
shell echo `pwd`  /home/t/src/ideas/non-rec/subdir
shell pwd         /home/t/src/ideas/non-rec/subdir
make: Entering directory `/home/t/src/ideas/non-rec/subdir'
../mk/Makefile:7: *** testing.  Stop.
make: Leaving directory `/home/t/src/ideas/non-rec/subdir'
```