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

A comparison on Linux (Debian) with bash follows. Don't have access to
Cygwin to test this. "linkedrec" is a symlink to "non-rec".

--Makefile--
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'
Merge branch 'master' of git://github.com/torpesco/nonrec-make into curdir

Since I've changed employer and I'm now full time under Linux there is
no reason to not merge this CURDIR improvement :).
Adding test directory for INCLUDES_$(d) usage
Fixing $(@RD) behaviour when TOP_BUILD_DIR is used.

Adding real_to_build_dir and build_to_real_dir and expressing some dirs
through it.
Merge pull request #13 from KholdStare/top-build-dir

Fix $(@RD) behaviour when TOP_BUILD_DIR is used