zlib: import zlib-1.3.1

This is vanilla zlib-1.3.1 imported over the existing zlib/ dir with:
* README adjusted to add the GCC note at the top;
* GCC's ChangeLog merged with the upstream one, as before;
* Deleted upstream Makefile as has been done before (we use an autoconf-
  generated one)

1 files changed, 0 insertions, 0 deletions

diff --git a/zlib/zlib.3.pdf b/zlib/zlib.3.pdf
new file mode 100644
index 00000000000..b224532bdd8
--- /dev/null
+++ b/zlib/zlib.3.pdf