Provide some xmake variables for install

[Freed-Wu]

Is your feature request related to a problem? Please describe.

Currently, luarocks build backend for xmake just run xmake install -o output, then install output/lib and lua. Perhaps we can provide more flexible method, provide some variables in xmake:

xmake.lua:

target("bar")
do
    set_prefixdir("$(prefix)/lib", { libdir = "foo" })
    add_rules("lua.module")
    add_files("*.c")
    add_installfiles("usage.md", {prefixdir= "$(prefix)/doc"})
    add_installfiles("example.conf", {prefixdir= "$(prefix)/conf"})
    add_installfiles("(foo/init.lua)", {prefixdir= "$(prefix)/lua"})
end

$ find .
lib/foo/bar.so
lua/foo/init.lua
conf/example.conf
doc/usage.md

Describe the solution you'd like

not directly load user's xmake.lua, we generate a xmake.lua and load it:

option("prefix")
do
    set_default("XXX")
end
includes"/the/path/of/user's/xmake.lua"

Then user can use $(prefix) or more xmake variables in their xmake.lua.

Describe alternatives you've considered

Or xmake support directly pass options. xmake f -D prefix=XXX user's/xmake.lua. like cmake -D.

Additional context

Here is similar thought for python backend.

[waruqi]

you can open a pr to add it.

build = {
    type = "xmake",
    variables = {
        xmake = {
            version = "2.5.1"
            configs = { -- custom option configs
                var1 = "xxx",
                var2.= "yyy"
            }
        }
    },
    copy_directories = {}
}

and modify this.

luarocks-build-xmake/src/xmake.lua

Line 499 in d96dfc9

local xmake_variables = build_variables.xmake or {}

local configs = xmake_variables.configs
for name, value in pairs(configs) do
    -- ...
end