Add unrtf package

UnRTF is a command-line program written in C which converts documents in Rich
Text Format (.rtf) to HTML, LaTeX, troff macros, and RTF itself.

Tested as working on Samsung Chromebook Plus (aarch64).

packages/unrtf.rb

+require 'package'
+
+class Unrtf < Package
+  description 'UnRTF is a command-line program written in C which converts documents in Rich Text Format (.rtf) to HTML, LaTeX, troff macros, and RTF itself.'
+  homepage 'https://www.gnu.org/software/unrtf/'
+  version '0.21.9'
+  source_url 'https://www.gnu.org/software/unrtf/unrtf-0.21.9.tar.gz'
+  source_sha256 '22a37826f96d754e335fb69f8036c068c00dd01ee9edd9461a36df0085fb8ddd'
+
+  binary_url ({
+  })
+  binary_sha256 ({
+  })
+
+  depends_on 'glibc'
+
+  def self.build
+    system './bootstrap'
+    system './configure',
+      "--prefix=#{CREW_PREFIX}",
+      '--disable-depedency-tracking',
+      '--disable-maintainer-mode'
+    system 'make'
+  end
+
+  def self.install
+    system 'make', "DESTDIR=#{CREW_DEST_DIR}", 'install'
+  end
+end