allow specifying python path (should probably better to let autoconf do this)

git-svn-id: svn://bind10.isc.org/svn/bind10/branches/f2f200910@257 e5f2f494-b856-4b98-b285-d166d9295462

src/bin/bind10/bind10.in

@@ -1,6 +1,6 @@
 #! /bin/sh
 
-PYTHON_EXEC=/usr/bin/python3.0
+PYTHON_EXEC={$PYTHON_EXEC:-/usr/bin/python3.0}
 BIND10_PATH=@abs_top_srcdir@/src/bin/bind10
 
 PATH=@abs_top_srcdir@/src/bin/msgq:@abs_top_srcdir@/src/bin/parkinglot:@abs_top_srcdir@/src/bin/bind-cfgd:$PATH