python = find_program(get_option('PYTHON'), required: true, native: true)
flex = find_program(get_option('FLEX'), native: true, version: '>= 2.5.35')
bison = find_program(get_option('BISON'), native: true, version: '>= 2.3')
-sed = find_program(get_option('SED'), 'sed', native: true)
+sed = find_program(get_option('SED'), 'sed', native: true, required: false)
prove = find_program(get_option('PROVE'), native: true, required: false)
tar = find_program(get_option('TAR'), native: true)
gzip = find_program(get_option('GZIP'), native: true)