Improve lookahead-threads auto selection Smarter decision to improve fast-first-pass performance in 2-pass encodes. Dramatically improves CPU utilization on multi-core systems.

Tested on a quad-core Ivy Bridge (12 threads, 1080p):
Fast first pass:
veryfast:     ~7% faster
faster:      ~11% faster
fast/medium: ~15% faster
slow/slower: ~42% faster
veryslow:    ~55% faster
CRF/1-pass:
veryfast:     ~9% faster
(all others remained the same)
64-bit cabac optimizations

WIN64:
~3% faster and 16 byte shorter cabac_encode_bypass
~8% faster cabac_encode_terminal
~4% faster PIC
Benchmarked on Ivy Bridge

UNIX64:
One instruction less in cabac_encode_bypass
x264.h: improve x264_encoder_reconfig documentation
x86inc: rename program_name to private_prefix Synced from libav. The new name is more descriptive and will allow defining a separate public prefix for externally visible library symbols.
x86inc: Add cvisible macro for C functions with public prefix

This allows defining externally visible library symbols.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
x86inc: Set ELF hidden visibility for global constants
Windows: Enable DEP and ASLR
Fix two bugs in predictor checking pmv wasn't checked properly in some cases, as well as zero vector. Output-changing portion of the following patch.
x86: optimize and clean up predictor checking Branchlessly handle elimination of candidates in MMX roundclip asm. Add a new asm function, similar to roundclip, except without the round part. Optimize and organize the C code, and make both subme>=3 and subme<3 consistent. Add lots of explanatory comments and try to make things a little more understandable. ~5-10% faster with subme>=3, ~15-20% faster with subme<3.