Make pg_xlogdump MSVC build work more like others.

Instead of copying xlogreader.c and *desc.c files into the source directory,
build them where they are. That's what we do for other binaries that need to
compile and link in files from elsewhere in the source tree.

The commit history suggests that it was done this way because of issues with
older versions of MSVC. I think this should work, but we'll see if the
buildfarm complains.