Import "context/detail/fcontext.hpp" instead of "context/all.hpp" when testing...

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
6 jobs for <span class="ref-name">patch-1</span> in 0 seconds (queued for 66 minutes and 14 seconds)
Status Job ID Name Coverage
  Test
failed #3828
amd64
job_clang_amd64

failed #3832
amd64
job_clang_amd64_nosh

failed #3830
amd64
job_clang_py2_amd64

failed #3827
amd64
job_gcc_amd64

failed #3831
amd64
job_gcc_amd64_nosh

failed #3829
amd64
job_gcc_py2_amd64

 
Name Stage Failure
failed
job_clang_amd64_nosh Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
job_gcc_amd64_nosh Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
job_clang_py2_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
job_gcc_py2_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
job_clang_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
job_gcc_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace