simplify CFLAGS handling for subdirectories

don't put paths into the include search path when they are not needed
don't auto-rebuild the AEL bison/flex output based on make dependencies (the generated files are already checked in, and the timestamps generated by a checkout will usually cause them to be overwritten)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@23582 65c4cc65-6c06-0410-ace0-fbb531ad65f3