Merge commit 'a7101eb40e69ada3872ec5aebe9c5c165745fb3a'

* commit 'a7101eb40e69ada3872ec5aebe9c5c165745fb3a':
  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>