* WT-2658 Only include PPC-specific files in PPC builds

* Add support for conditional file inclusion by adding a second argument on lines in dist/filelist.  Add a new automake conditional, POWERPC_HOST, set for the various PPC host CPUs.  Change dist/filelist to only compile the checksum/power8 files if POWERPC_HOST is set.

* Merge the Windows and POSIX file lists, use POSIX_HOST and WINDOWS_HOST to mark source files needed for builds on those systems.