Also some dead links were repaired or removed. Some http links
were left as is where attempting https did not work, or where it
is a historical record.
This moves the once code from crc32.c to zutil.c, and uses it also
for building the inflate fixed tables when BUILDFIXED is defined.
The fixed tables are now housed in inftrees.c, shared by inflate.c
and infback.c. The once() function is now external, and so is
renamed to z_once() to avoid name collisions.
If either BUILDFIXED or DYNAMIC_CRC_TABLE is defined, and atomics
are not available, then a warning is issued noting that zlib is not
thread-safe.
ZLIB_INSECURE must be defined in order to compile code that uses
the insecure functions vsprintf() or sprintf(). This would occur
only if the standard vsnprintf() or snprintf() functions are not
available. Providing the --insecure option to ./configure will
define ZLIB_INSECURE. A flag is added to zlibCompileFlags() to
indicate that gzprintf() is not implemented due to the need for
the use of an insecure function, but ZLIB_INSECURE was not
defined.
Point to a modern, well-supported implementation, for use when the
compiler chain in use does not provide those functions, and a
secure alternative to sprintf/vsprintf for the gz functions is
desired.
