Support using external pkcs11 headers

[randombit]

Debian thinks the OASIS IPR statement on the pkcs11 headers is ambiguous and so deletes the entire pkcs11 submodule. :/ This ends up being the case also in Ubuntu and other downstream distros.

Not really sure offhand how this will work but if there is a configure flag eg --with-pkcs11-headers=(internal|p11-kit|...) we might be able to avoid this situation. I suspect Debian will still want to be able to rm -rf the actual pkcs11.h before distribution, so the build system would have to be extended to handle the case of "include this file if it exists but accept it being missing".

[randombit]

#4540 switches to using public domain pkcs11 headers which addresses the issue without extra build complications. Closing