Fix a problem that cuased AST_DATA_DIR in defaults.h to be empty. The cause is

that since ASTDATADIR is explicitly exported using "export ASTDATADIR" at the
top of the Makefile, make no longer considers the variable "undefined", so the
Makefile can't use ?= to set ASTDATADIR if not yet set.
(issue #8063, reported by akohlsmith, fixed by me)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@44125 65c4cc65-6c06-0410-ace0-fbb531ad65f3

Makefile

@@ -109,7 +109,9 @@ else
   ASTVARLIBDIR=$(localstatedir)/lib/asterisk
 endif
 endif
-ASTDATADIR?=$(ASTVARLIBDIR)
+ifeq ($(ASTDATADIR),)
+  ASTDATADIR:=$(ASTVARLIBDIR)
+endif
 
 # Asterisk.conf is located in ASTETCDIR or by using the -C flag
 # when starting Asterisk