Firenzina 2.3.1 xTreme

Redundant checks eliminated in eval.c and material_value.c. Prefetch
code by Quoc Vuong added to fire.h and make_move.c. Linux compatibility
is yet to be verified (and restored if necessary).
Prefetch Linux compatibility fix by ZirconiumX

_builtin_prefetch is the Linux equivalent of _mm_prefetch. Now Firenzina
2.3.1 xTreme compiles under Windows and Linux.
Bug fix in material_value.c

An if is moved to its "right" place. A couple of redundant checks are
eliminated.
Fixing fire.h and material_value.c

Small, but useful fixes.
Parameter tuning, Min_Threads added

Set chess piece values to those from the GH compile of Fire 2.2.
Introduce Min_Threads and remove doubling of the CPU number to get the
number of threads. Now it is still possible to have more threads than
processors if one wants to, but such a setting is no longer the default,
which was bad.
Revert an earlier change to a BenchMark call.

Very minor housekeeping: Revert an earlier change to a BenchMark call
argument.
Minor middle-of-release fix

This change to make_move.c has no impact on the specialized compiles
released so far. It makes sure that prefetch is used only if HasPreFetch
is set. Relevant for making generic profiles.
Another minor middle-of-release fix

No impact on specialized compiles released earlier. This change in
fire.h and win64bits.h makes sure that _mm_popcnt_u64 is used only if
HasIntrinsics is defined. Relevant to generic compile generation.