Apply suggestion from Andrew for srcdir functionality.

git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@1533 fd59a12c-fef9-0310-b244-a6a79926bd2f

exec/Makefile

@@ -32,7 +32,7 @@
 
 # Include configuration
 #
-srcdir:=$(CURDIR)/../
+srcdir ?= $(CURDIR)/../
 
 include $(srcdir)Makefile.inc