Add macro GPGRT_INLINE and avoid -Wundef warnings

* src/gpg-error.h.in (GPG_ERR_INLINE): Use #if defined for possible
undefined macros to avoid warnign with GCC's -Wundef option.
(GPGRT_INLINE): New.

I still consider using -Wundef for regular building a bad behaviour
because undefined macros have a well defined value in a cpp
conditional. That warning is useful for debugging build problems but
should not be used as standard warning option. Anyway, here is a fix.