Remove __P() macro for portability

[orlitzky]

The __P() macro used in e.g. https://github.com/trusteddomainproject/OpenDKIM/blob/5c539587561785a66c1f67f720f2fb741f320785/librbl/rbl.h#L62 existed only to support ancient compilers. The macro is a no-op in modern glibc,

/* These two macros are not used in glibc anymore.  They are kept here          
   only because some other projects expect the macros to be defined.  */
#define __P(args)       args
#define __PMT(args)     args

and are missing entirely in musl. So eliminating them would help make OpenDKIM build on musl without hurting anyone else (famous last words).

[thegushi]

Those famous last words are the ones that concern me. Let me discuss this internally.