Fix m4 recipe for boost::context to work with boost >=1.68

Import context/detail/fcontext.hpp instead of context/all.hpp when testing for the presence of boost::context.

context/all.hpp has been removed in recent versions of boost::context.

See:

• https://github.com/boostorg/context/issues/81
• https://github.com/boostorg/context/commit/2e37599461912cc1679f106a25456a493f7f0b27