I am pretty sure this isn't possible. The headers get overridden by the most local settings. There isn't a way to concatenate them. In some cases there isn't a way to figure out what you want, e.g. if a heading property said ":tangle no" and your header said ":tangle yes" it would not make sense to concatenate these to ":tangle no yes".

I think you have to add -Wall to the src header.

John

-----------------------------------
Professor John Kitchin 
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803

On Tue, Mar 6, 2018 at 1:25 AM, Michael Welle <mwe012008@gmx.net> wrote:
Hello,

how can I concatenate header args? Let's assume I have an Org structure
like follows:

* foo
 :PROPERTIES:
 :header-args: :flags -Wall
 :END:
** bar
#+begin_src C :flags -lm
#+end_src

Now I want the code to be compiled with -Wall _and_ -lm.

Regards
hmw