add static_cast to get rid of warnings for intel compiler on linux platform

Fix get_git_version to use GIT_EXECUTABLE.

get_git_version CMake function uses 'git' command directly, instead of
GIT_EXECUTABLE variable. This causes CMake errors while generating
project files in environments, where 'git' command is not present
in PATH.

Fixes #144  CMake fails without Git 

Android fix

The sane thing here would be to use std::stoul but this is not available in the android-ndk...

- Added missing header to test/benchmark_test.cc
- Changed std::stoul to std::atol in test/filter_test.cc because of a
  limitation in the android-ndk (http://stackoverflow.com/questions/17950814/how-to-use-stdstoul-and-stdstoull-in-android)

sys/sysctl.h is not available in the android-ndk.

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