added --with-xml2-config / --without-xml2-config options to extconf.rb (original behavior preserved if neither is specified, or using --without-xml2-config)
added --with-xml2-config / --without-xml2-config options to extconf.rb (original behavior preserved if neither is specified, or using --without-xml2-config)

sferik/libxml-ruby,increments/libxml-ruby,xml4r/libxml-ruby,sferik/libxml-ruby,increments/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,xml4r/libxml-ruby,xml4r/libxml-ruby,sferik/libxml-ruby,increments/libxml-ruby,sferik/libxml-ruby