Add GPL license files to .gitignore and gemspec

Author: kojix2

Diff for: .gitignore

@@ -17,4 +17,6 @@ mkmf.log
 /.vscode/
 
 /ext/alglib/*.cpp
-/ext/alglib/*.h
+/ext/alglib/*.h
+gpl2.txt
+gpl3.txt

Diff for: Rakefile

@@ -48,7 +48,9 @@ namespace :ext do
         when '.cpp', '.h'
           FileUtils.mkdir_p File.dirname(path_to_extract)
           entry.extract(path_to_extract) { true }
-        end
+	when '.txt' # LICENCE gpl2.txt gpl3.txt
+	  entry.extract(File.basename(entry.name)){ true }
+	end
       end
     end
 

Diff for: alglib4.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/kojix2/ruby-alglib'
   spec.license       = 'MIT'
 
-  spec.files         = Dir['{ext,vendor}/**/*', 'lib/**/*.rb', 'LICENSE.txt', 'README.md']
+  spec.files         = Dir['{ext,vendor}/**/*', 'lib/**/*.rb', '*.txt', 'README.md']
   spec.require_paths = ['lib']
   spec.extensions    = ['ext/alglib/extconf.rb']