Additionally, drop non-GNU compiler support entirely.
Improve compiling documentation a little bit.

Passed non-regression STC:

Elo   | 7.92 +- 6.24 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.96 (-2.94, 2.94) [-4.00, 1.00]
Games | N: 4740 W: 1000 L: 892 D: 2848
Penta | [67, 476, 1180, 576, 71]
http://chess.grantnet.us/test/35944/

Bench: 3,740,491