• add check_builtin
• perltidy code and add perltidy test (taken from MooX::Options)
• ensure for many check_* methods that externally controlled ac_av_* environment variables are correctly passed into config.h
• fix check_dirent_header to cache proving header and proving availability of 'DIR *' in such a header separately
• _get_extra_linker_flags: add library dirs
• enable test

Passed the make tests.