Takes the environment variable DESTDIR in rake install

If the environment variable `DESTDIR` is set, the writing destination is `<DESTDIR>/<PREFIX>/...`.
It is assumed to be used for package work.

doc/guides/compile.md

@@ -595,6 +595,9 @@ The `PREFIX` environment variable affects all build targets and changes the `/us
 The `MRuby::Build#install_prefix` can be set for each individual build target.
 In this case, the environment variable `PREFIX` is ignored.
 
+Also, if the environment variable `DESTDIR` is set, it will prepend to the path obtained by `install_prefix` to determine the final write directory.
+This is intended for temporary file expansion by the user's package work.
+
 ---
 
 To summarize:
@@ -602,6 +605,7 @@ To summarize:
 - The default value of the environment variable `PREFIX` is `/usr/local`.
 - For the "host" build target, the default value of `MRuby::Build#install_prefix` is `<PREFIX>`.
 - For a build target other than "host", the default value of `MRuby::Build#install_prefix` is `<PREFIX>/mruby/<build-name>`.
+- If the environment variable `DESTDIR` is set, the actual write directory is `<DESTDIR>/<MRuby::Build#install_prefix>`.
 
 ## Tips
 

lib/mruby/build.rb

@@ -9,6 +9,7 @@ module MRuby
   autoload :Presym, "mruby/presym"
 
   INSTALL_PREFIX = ENV['PREFIX'] || ENV['INSTALL_PREFIX'] || '/usr/local'
+  INSTALL_DESTDIR = ENV['DESTDIR'] || ''
 
   class << self
     def targets

tasks/install.rake

@@ -13,7 +13,7 @@ task "install:bin"
 MRuby.each_target do |build|
   next if build.internal?
 
-  prefix = build.install_prefix
+  prefix = File.join(MRuby::INSTALL_DESTDIR, build.install_prefix)
 
   task "install:full" => "install:full:#{build.name}"