make use of make variable CURDIR instead of calling pwd

make already provides the current working directory in a variable, so make
use of it instead of forking a shell. Also replace usage of PWD by
CURDIR. PWD is provided by most shells, but not all, so this makes the
build system more robust.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Michal Marek <mmarek@suse.com>
diff --git a/tools/gpio/Makefile b/tools/gpio/Makefile
index 250a891..b440153 100644
--- a/tools/gpio/Makefile
+++ b/tools/gpio/Makefile
@@ -3,7 +3,7 @@
 bindir ?= /usr/bin
 
 ifeq ($(srctree),)
-srctree := $(patsubst %/,%,$(dir $(shell pwd)))
+srctree := $(patsubst %/,%,$(dir $(CURDIR)))
 srctree := $(patsubst %/,%,$(dir $(srctree)))
 endif