Adds SSE2 optimized hash shifting to fill_window.

Uses SSE2 subtraction with saturation to shift the hash in
16B chunks. Renames the old fill_window implementation to
fill_window_c(), and adds a new fill_window_sse() implementation
in fill_window_sse.c.

Moves UPDATE_HASH into deflate.h and changes the scope of
read_buf from local to ZLIB_INTERNAL for sharing between
the two implementations.

Updates the configure script to check for SSE2 intrinsics and enables
this optimization by default on x86. The runtime check for SSE2 support
only occurs on 32-bit, as x86_64 requires SSE2. Adds an explicit
rule in Makefile.in to build fill_window_sse.c with the -msse2 compiler
flag, which is required for SSE2 intrinsics.
add SSE4.2 optimized hash function

For systems supporting SSE4.2, use the crc32 instruction as a fast
hash function. Also, provide a better fallback hash.

For both new hash functions, we hash 4 bytes, instead of 3, for certain
levels. This shortens the hash chains, and also improves the quality
of each hash entry.
add PCLMULQDQ optimized CRC folding

Rather than copy the input data from strm->next_in into the window and
then compute the CRC, this patch combines these two steps into one. It
performs a SSE memory copy, while folding the data down in the SSE
registers. A final step is added, when we write the gzip trailer,
to reduce the 4 SSE registers to 32b.

Adds some extra padding bytes to the window to allow for SSE partial
writes.
crc_folding
add ZLIB_LEVEL environmental variable

The goal is to provide a way for a system administrator
to easily override the deflate level. This environmental
variable is checked in deflateInit, and takes precedence
over the level set by the api.
deflate: add new deflate_quick strategy for level 1

The deflate_quick strategy is designed to provide maximum
deflate performance.

deflate_quick achieves this through:
    - only checking the first hash match
    - using a small inline SSE4.2-optimized longest_match
    - forcing a window size of 8K, and using a precomputed dist/len
      table
    - forcing the static Huffman tree and emitting codes immediately
      instead of tallying

This patch changes the scope of flush_pending, bi_windup, and
static_ltree to ZLIB_INTERNAL and moves END_BLOCK, send_code,
put_short, and send_bits to deflate.h.

Updates the configure script to enable by default for x86. On systems
without SSE4.2, fallback is to deflate_fast strategy.
deflate: add new deflate_medium strategy

As the name suggests, the deflate_medium deflate strategy is designed
to provide an intermediate strategy between deflate_fast and deflate_slow.
After finding two adjacent matches, deflate_medium scans left from
the second match in order to determine whether a better match can be
formed.