what was adopted in tzcode95c to solve the problem of the first char
detzcode() extracts requiring sign extention (this is needed for
machines with 64 bit longs).
Our implementation is a bit more efficent, but requires a new macro,
SIGN_EXTEND_CHAR(), and a conditional to set it appropriately).
One minor enhancement for machines without ANSI compilers that have 32
bit longs would be to define SIGN_EXTEND_CHAR() to be the identity
macro.