zlib version should not be hardcoded in etc.c.zlib

[dlangBugzillaToGithub]

joseph.wakeling (@WebDrake) reported this on 2016-06-05T18:15:19Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=16128

Description

The etc.c.zlib module includes hardcoded copies of 2 #define'd constants from zlib.h:

    const char[] ZLIB_VERSION = "1.2.8";
    const ZLIB_VERNUM = 0x1280;

However, the builder (or user) of phobos has no guarantee that this is the version installed on their system; and for libphobos packages built with dynamic linking against zlib (as e.g. on Debian/Ubuntu), there is no guarantee that the installed zlib version will not be upgraded.

It therefore seems dangerous/misleading to have these constants in the D wrapper, unless it is to specify a minimum supported version of zlib.

If we assume/enforce static linking, then the version constants could be autogenerated at build time, but this seems too strong an assumption given the existing Debian/Ubuntu packaging situation.

Note also that these version constants are missing several of their C header-defined equivalents:

    #define ZLIB_VERSION "1.2.8"
    #define ZLIB_VERNUM 0x1280
    #define ZLIB_VER_MAJOR 1
    #define ZLIB_VER_MINOR 2
    #define ZLIB_VER_REVISION 8
    #define ZLIB_VER_SUBREVISION 0