build/Makefile.docbook: fix INC_DIR for openbsc

When Makefile.docbook.inc gets included by openbsc, the PWD Makefile
variable is empty (probably because the including file is not generated
by automake). Use the shell pwd command instead.

This fixes the master-openbsc jenkins job which has been failing since
previous patch dba860c7 ("VTY references: set git version and date")
was merged.

Related: OS#4063
Change-Id: I6026623ec9a5eef0ca8c47bf46605086e361c2bc
---
M build/Makefile.docbook.inc
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/build/Makefile.docbook.inc b/build/Makefile.docbook.inc
index eff6c66..df17646 100644
--- a/build/Makefile.docbook.inc
+++ b/build/Makefile.docbook.inc
@@ -28,7 +28,7 @@
 all: $(DOCBOOKS_PDF)
 
 # Allow the users to re-define the include directory
-INC_DIR ?= $(PWD)/generated
+INC_DIR ?= $(shell pwd)/generated
 
 # Lint the file
 %.xml-lint: %.xml

