Improve to configure working and build directories

[waruqi]

xmake will support two ways of configuring working directory.

Introduction

Working directory

The current working directory where xmake is running, and it is also the directory where the project cache configuration .xmake is stored.

- workdir
  - .xmake
  - build

Build directory (-o/--buildir)

Build the output directory of the build artifacts.

build
└── macosx
    └── x86_64
        └── release
            ├── demo

Project directory (-P/--project)

The root directory of the project's source code, which is also the directory containing the root xmake.lua file.

- projectdir
  - xmake.lua

Internal working directory mode (default)

This is the behaviour of the existing version, which has never changed.

The working directory is the project root directory by default.

- projectdir (workdir)
  - build (generated)
  - .xmake (generated)
  - src
  - xmake.lua

$ cd projectdir
$ xmake

Set the other build directory outside the working directory

we can set the given build directory, but the working directory is still the project root directory.

- projectdir (workdir)
  - .xmake (generated)
  - src
  - xmake.lua
- build

$ cd projectdir
$ xmake f -o ../build

External working directory mode (new)

We can use -P projectdir to set the given project directory and it will enable external working directory mode.

Lock the current directory as the working directory, and xmake will cache the specified project directory.

The working directory is the current directory by default.

- workdir
  - build (generated)
  - .xmake (generated)
- projectdir
  - src 
  - xmake.lua

$ cd workdir
$ xmake f -P ../projectdir
$ xmake

Set the other build directory outside the working directory

- build (generated)
- workdir
  - .xmake (generated)
- projectdir
  - src 
  - xmake.lua

$ cd workdir
$ xmake f -P ../projectdir -o ../build

[waruqi]

$ xmake update -s dev
$ cd workdir
$ xmake f -P ../projectdir
$ xmake
...

[waruqi]

@davidchisnall you can try it, I have merged into dev branch.

[davidxiaozhi]

帮忙测试验证
xmake.lua 存在且能执行 xmake 构建

更新 xmake 之后执行 xmake f -P ../projectdir 提示重新生成 xmake.lua