The libraries included in the external directory are not supported by the TNG developers.

zlib: version 1.2.8, with content not necessary for TNG build removed.

Modifications within source files were
* minor refactoring in crc32.c to make clear that some code should sometimes be unreachable
* replacing some undefined behaviour (-1L << 16) with well defined value (~0xFFFFL) in inflate.c
* commenting out unused zlibCompileFlags() declaration and definition
* removing unnecessary configurability from zconf.h
* removing #include of unused gzguts.h
