build: fix Makefile wrt finding node executable

Not all shells set PWD, so use CURDIR instead.

`which node` may return the empty string, so guard against
that too. Consider:
if [ -x `which velociraptor` ] && [ -e `which velociprator` ];\
then echo "run"; else echo "keep calm"; fi;

PR-URL: nodejs#18040
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

Makefile

@@ -14,6 +14,7 @@ COVTESTS ?= test-cov
 GTEST_FILTER ?= "*"
 GNUMAKEFLAGS += --no-print-directory
 GCOV ?= gcov
+PWD = $(CURDIR)
 
 ifdef JOBS
   PARALLEL_ARGS = -j $(JOBS)
@@ -640,7 +641,7 @@ out/doc/api/assets/%: doc/api_assets/% out/doc/api/assets
 available-node = \
   if [ -x $(PWD)/$(NODE) ] && [ -e $(PWD)/$(NODE) ]; then \
 		$(PWD)/$(NODE) $(1); \
-	elif [ -x `which node` ] && [ -e `which node` ]; then \
+	elif [ -x `which node` ] && [ -e `which node` ] && [ `which node` ]; then \
 		`which node` $(1); \
 	else \
 		echo "No available node, cannot run \"node $(1)\""; \