Fixes compilation issue on Macs

ba141ac0 renamed OS_MACOSX -> BENCHMARK_OS_MACOSX,
except for an include guard in src/cycleclock.h

Visual studio warnings fix

This reverts commit 0a2bd8935391e42f2581ec4826f7ac5f8fc9041a.

This also points out some important issues like:
warning: conversion to 'size_t {aka unsigned int}' from 'long long int' may alter its value [-Wconversion]
   state.SetBytesProcessed(items_processed * sizeof(v));
This occurs on 32 bit systems where size_t is only 32 bits.

Unary minus where applied to an unsigned type.

Fix for Ubuntu & Clang & C++14

Fix issue #141. Use size_t instead of int for the iteration count

Remove std::is_trivially_destructible assertion. Fix issue #134.

Updates Initialize() to work with an argv as 'char**'

Jleroy/vs2013 compatibility

Fix documentation comment in benchmark_api.h w.r.t CustomArguments

Fix doc sample typos in header and README

* iterations() is a method
* int64_t fix in a couple of places