Clarify when forcing the moves loop

In some cases we want to go direcly to the moves loop
without checking for early return. The patch make this
logic more clear and consistent.

Tested for no regression, passed STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 25282 W: 5136 L: 5022 D: 15124

and LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 72007 W: 12133 L: 12095 D: 47779

bench: 9316798
Assorted nitpicking code-style

No functional change.
Retire 'os' flag from Makefile

Appears to be unused

No functional change

Resolves #147
Fix profile build for syzygy

Touch source files under syzygy directory to force recompilation
after collecting profile data.

No functional change

Resolves #149
MSVC compiling fixes

No functional change.

Resolves #150
Halve StormDanger bonus for blocked pawn on A/H file

STC
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 3410 W: 758 L: 641 D: 2011

LTC
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 33225 W: 5708 L: 5445 D: 22072

Bench: 8465811

Resolves #153
Only use _ReadWriteBarrier on MSVC

It was causing compile errors when cross-compiling using mingw.

No functional change.