boost.m4 check for boost_thread dependency for boost_context broken

[James-TR]

The boost.m4 check for linking boost_thread to boost_context is incorrect, as this isn't linked where is available. The first release to change this behaviour is 1.61.0

Here's the relevant commit:
boostorg/context@01d3c90

Here's the relevant lines in the 1.71.0 release of boost_context:
https://github.com/boostorg/context/blob/boost-1.71.0/build/Jamfile.v2#L783-L789

[James-TR]

The check is here https://github.com/tsuna/boost.m4/blob/master/build-aux/boost.m4#L670-L675

[James-TR]

I've found an example of how to check for the mutex std::call_once here https://github.com/cburschka/lyx/blob/44bbd0b0ef8450f4d88dfb8c3345118f2fd14dfc/config/lyxinclude.m4#L252-L275

[James-TR]

Just thought I would clarify that the current check will still be required for versions from 157 -> 160 as BOOST::THREAD was blindly included. From 161 and above the check was fixed to implement the check for mutex std::call_once