fix permission issue on some environment

Rakefile

@@ -1,6 +1,7 @@
 require 'rubygems'
 require 'rubygems/package_task'
 require 'rake/testtask'
+require "rake/extensiontask"
 
 spec = eval(File.new("wkhtmltopdf-binary.gemspec").readlines.join("\n"))
 
@@ -14,3 +15,6 @@ end
 
 desc 'Run tests'
 task default: :test
+
+Rake::ExtensionTask.new "wkhtmltopdf" do |ext|
+end

ext/wkhtmltopdf/extconf.rb

@@ -0,0 +1,45 @@
+require 'mkmf'
+require 'rbconfig'
+require 'zlib'
+
+suffix = case RbConfig::CONFIG['host_os']
+         when /linux/
+           os = `. /etc/os-release 2> /dev/null && echo ${ID}_${VERSION_ID}`.strip
+
+           if os.start_with?('elementary') || os.start_with?('linuxmint') || os.start_with?('pop')
+             os = 'ubuntu_18.04'
+           end
+
+           # CentOS 6 doesn't have `/etc/os-release`.
+           if (os.empty? && File.read('/etc/centos-release').start_with?('CentOS release 6')) ||
+               os.start_with?('amzn_')
+             os = 'centos_6'
+           end
+
+           # Deepin fallback
+           os = 'debian_9' if /deepin/.match?(os)
+
+           architecture = RbConfig::CONFIG['host_cpu'] == 'x86_64' ? 'amd64' : 'i386'
+
+           "#{os}_#{architecture}"
+         when /darwin/
+           'macos_cocoa'
+         else
+           'unknown'
+         end
+
+binary = File.join(File.dirname(__FILE__), "../../bin/wkhtmltopdf_#{suffix}")
+puts binary
+if File.exist?("#{binary}.gz") && !File.exist?(binary)
+  File.open binary, 'wb', 0o755 do |file|
+    Zlib::GzipReader.open("#{binary}.gz") { |gzip| file << gzip.read }
+  end
+end
+
+unless File.exist? binary
+  raise 'Invalid platform, must be running on Ubuntu 16.04/18.04/20.04 ' \
+        'CentOS 6/7/8, Debian 9/10, or intel-based Cocoa macOS ' \
+        "(missing binary: #{binary})."
+end
+
+create_makefile 'wkhtmltopdf/wkhtmltopdf'

wkhtmltopdf-binary.gemspec

@@ -11,6 +11,7 @@ Gem::Specification.new do |s|
   s.files = Dir['bin/*']
   s.executables << "wkhtmltopdf"
   s.require_path = '.'
+  s.extensions = %w[ext/wkhtmltopdf/extconf.rb]
 
   s.add_development_dependency "minitest"
 end