Factor out pawn attacks in 'lever'

Improves readibility and possibly speed.

No functional change.
Triviality in UCI::loop

Code style paranoid in action here :-)

No functional change.
Fix a warning with MSVC in 'analyze' mode

Here MSVC is worried that

StepAttacksBB[PAWN][psq]

could overflow, so change psq initialization
to clarify psq is always less than 64.

No functional change.
Remove -ansi flag for Clang

It seems this flag is only for gcc and
yields a warning under OSX Mavericks:

clang: warning: argument unused during compilation: '-ansi'

No functional change.