There are a very few to the source code of zlib-1.2.1 used here.

- a bug fix in deflate.c (picked up by the use of valgrind).
- zconf.h includes R's config.h
- gzio.c has been amended to allow LFS using fseeko/ftello where these
  are supported.

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.