Namespace the bundled zlib symbols, to avoid clash with user zlib

Namespace the bundled zlib symbols, to avoid clash with user zlib

When Qt is being compiled and is using the bundled zlib, QtCore needs
to export the zlib symbols, since zlib is needed in other Qt
libraries as well. That gives a danger of a potentially disastrous
symbol clash if the user later on links with both Qt and an external
zlib (ref. e.g. QTBUG-15071).

This commit enables a zlib compilation flag that causes all zlib
symbols to be redefined with a prefix. Hence, they will not clash
with a standard zlib.

A minor drawback is that zlib.h will now have #defines for a few
semi-normal identifiers. Hence, a couple of more changes are done:
In the private qzip code, the identifer crc32 had to be renamed.
QHttpNetworkReplyPrivate needed no change, but as a defensive measure
the #include <zlib.h> is moved from the _p.h file to the .cpp file,
to avoid surprising compilation errors later in code that include
that header.

This commit does not in itself solve the issue of how to let Qt
libraries outside of qtbase use the same bundled zlib, but it is
a prerequisite for that.

Change-Id: If84105901a8c90d35009faffe660c85a6bd2fee5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>