Merge commit 'a7101eb4'

* commit 'a7101eb4':
  configure: Simplify some library checks via check_lib()

This commit is a noop as it doesn't apply to FFmpeg:
- the nanosleep check doesn't use add_extralibs in FFmpeg
- the pthread checks check multiple functions before add the extra
  libraries
Merged-by: Clément Bœsch <u@pkh.me>