Improve ZBLOCK handling with zlib < 1.2.4

Improve ZBLOCK handling with zlib < 1.2.4

The original commit for this issue (62b1293) assumed Z_BLOCK was
only defined in < 1.2.4. However, this flush type *is* defined but
is only unavailable for use with deflate().

This new commit correctly checks the ZLIB_VERNUM constant to
determine if Z_BLOCK flush is available for the current deflate()
operation and triggers an appropriate error as needed.

New ZLIB_VERSION and ZLIB_VERNUM constants are also exposed in
userland to allow testing this behavior in environments running
zlib < 1.2.4 (ZLIB_VERNUM check is needed).