add xmake.lua file to make it easier to build libuv

[waruqi]

Hi, I found that libuv uses different build systems to build for different platforms. So I wrote a xmake.lua file to make it easier to build libuv.

We can use the following command to build and run tests.

For the current host platform (windows/macos/linux):

$ xmake
$ xmake run test

For the android platform:

$ xmake f -p android --ndk=/home/android-ndk-r10e
$ xmake

For the iphoneos platform:

$ xmake f -p iphoneos -a [arm64|armv7]
$ xmake

For the watchos platform:

$ xmake f -p watchos
$ xmake

For the cross toolchains platform:

$ xmake f -p cross --sdk=/home/xxx/toolchains/sdk
$ xmake

For the other platform(aix/sunos/os390/freebsd/openbsd/...):

$ xmake f -p [aix|sunos|os390|...] --sdk=/home/xxx/toolchains/sdk --cross=.. 
$ xmake

More cross compilation info, please see xmake.io and cross-compilation

Or generate vs project file and makefile:

$ xmake project -k vs2017 -m "debug,release"
$ xmake project -k makefile

If you feel that this does not help the libuv, please ignore this PR. 😄

[saghul]

Hi there! Welcome to libuv!

So, a similar thing happened yesterday, when someone submitted a gradle build system. Here is the answer: #1462 (comment)

TLDR: we have enough build systems, sorry. Please do reach out next time before working on such things so you can avoid doing the work if it's not going to be merged.