Something must be wrong with the assumptions made on the <zlib.h> provided by the OS. On my CentOS 4.6, the "gz_header" type is not provided:
../generic/tclZlib.c:51: error: syntax error before '*' token
../generic/tclZlib.c:53: error: syntax error before "gz_header"
Prio 9 since I can no longer compile Tcl :(