Insert platform dependent typedefs into gpgme.h

We already modify gpgme.h per playform and thus we can also get rid of
some #ifdefs. The change does not change anything for current
platforms but should do the right think for W64.

Note that as per MS specs ssize_t is to be defined as LONG_PTR which
translates to a long on 32 bit platforms and to __int64 on 64 bit
Windows platforms. We already used long in the past. There seems to
be a problem with some versions of mingw32 which includes a ssize_t
type typedefed to int. O(n 32 bit W32 platforms int and long are
identically.