Temporary fork a copy of libz for capabilities.

Once it's stable I'll merge with libz.
Actually take a value folowing -B.
Set CC to CHERI_CC a more firmly.
if 0 out an unused function that newer clang versions warn about.
Fix a broken header guard.
Build tcpdump static and use CHERI_CC.
Compiling, but untested capability aware libz.

Todo:
 - Rename capability aware functions and implement capability unaware,
   ABI compatible wrappers.
 - Add sandboxing.