ZLIB is mandatory

[dmikushin]

Thank you for sharing your valuable development with public. Please note the ZLIB presence is crucial for normal Tcl operation. Without zlib, Tcl uses zlib API stubs, and all kind of weird things start to happen, e.g. like:

alloc: invalid block: 0x9319b70: c0 9
Aborted

This is controlled mainly by add_definitions(-DHAVE_ZLIB=1), preceded by find_package(ZLIB REQUIRED) on Linux and appropriate additional treatment on Windows.