Multithreading advice on the manual

[Miguel Ángel Arruga Vivas]

Hi Bruno and everybody,

I've been reading the gettext manual and I've found an unfortunate
passage here[1].  The multithreaded example contains a race condition
(volatile doesn't mean atomic) and a TOCTOU error even after changing to
an atomic variable.  The implementation of this pattern is what
(pthread_)call_once does, so instead of discouraging its use I propose
promoting it.

The attached patch documents the modern usage of once_flag/call_once
with a link to GNU libc manual and modifies the paragraph below the
example to suggest either pthread_once or any other equivalent primitive
provided by the system.

What do you think?

Best regards,
Miguel

[1] https://www.gnu.org/software/gettext/manual/html_node/Libraries.html

0001-Update-manual-advice-on-multiple-threads.patch
Description: Manual Update v1