Add libunbound 1.6.2

packages/libunbound.rb

+require 'package'
+
+class Libunbound < Package
+  version '1.6.2'
+  source_url 'https://www.unbound.net/downloads/unbound-1.6.2.tar.gz'
+  source_sha1 'de370b1ac8e260db9c4c1504453752713dd8818f'
+
+  depends_on 'flex' => :build
+  depends_on 'bison' => :build
+  depends_on 'gawk' => :build
+  depends_on 'expat'
+
+  def self.build
+    system "./configure", "--enable-shared", "--disable-static", "--with-pic"
+
+    # flex 2.6.3 requires -P option to rename yylex and other funcions
+    system "sed", "-i", "Makefile", "-e", '/$(LEX) -t $(srcdir)\/util\/configlexer.lex/s:-t:-t -Pub_c_:'
+
+    system "make"
+  end
+
+  def self.install
+    system "make", "DESTDIR=#{CREW_DEST_DIR}", "install"
+  end
+
+  def self.check
+    system "make", "test"
+  end
+end