Add libsass package (#3725)

* Add libsass package

Libsass is a C/C++ port of the Sass engine.

Tested on ARM.

* Abstract libdir using const

packages/libsass.rb

+require 'package'
+
+class Libsass < Package
+  description 'LibSass is a C/C++ port of the Sass engine'
+  homepage 'https://sass-lang.com/libsass'
+  version '3.6.3'
+  source_url 'https://github.com/sass/libsass/archive/3.6.3.tar.gz'
+  source_sha256 'dadb470bb9141e91b437119d367654427180ed2b3d04b8000eab5b0ca47cd5bb'
+
+  binary_url ({
+  })
+  binary_sha256 ({
+  })
+
+
+  def self.build
+    system 'autoreconf', '-i'
+    system './configure', "--prefix=#{CREW_PREFIX}", "--libdir=#{CREW_LIB_PREFIX}"
+    system 'make'
+  end
+
+  def self.install
+    system 'make', "DESTDIR=#{CREW_DEST_DIR}", 'install'
+  end
+end