There are a very few to the source code of zlib-1.2.3 used here.

- zconf.h includes R's config.h
- gzio.c has been amended to allow LFS using fseeko/ftello where these
  are supported, and fseek64o/ftell64o on Windows.

Makefile.in is ours, not zlib's, and allows us to use PIC flags so this
can be included in libR.so.

We assume ISO C, so assume errno.h exists.
