[Boost-build] how dose long double support determined

From: Xspeed (xspeed1989_at_[hidden])
Date: 2019-07-07 05:15:29

  Performing configuration checks

    - default address-model    : 32-bit
    - default architecture     : x86
    - C++11 mutex              : no
    - Boost.Config Feature Check: cxx11_auto_declarations : no
    - Boost.Config Feature Check: cxx11_constexpr : no
    - Boost.Config Feature Check: cxx11_defaulted_functions : no
    - Boost.Config Feature Check: cxx11_final : no
    - Boost.Config Feature Check: cxx11_hdr_mutex : no
    - Boost.Config Feature Check: cxx11_hdr_tuple : no
    - Boost.Config Feature Check: cxx11_lambdas : no
    - Boost.Config Feature Check: cxx11_noexcept : no
    - Boost.Config Feature Check: cxx11_nullptr : no
    - Boost.Config Feature Check: cxx11_rvalue_references : no
    - Boost.Config Feature Check: cxx11_template_aliases : no
    - Boost.Config Feature Check: cxx11_thread_local : no
    - Boost.Config Feature Check: cxx11_variadic_templates : no
    - has_icu builds           : no
warning: Graph library does not contain MPI-based parallel components.
note: to enable them, add "using mpi ;" to your user-config.jam
    - zlib                     : no
    - bzip2                    : no
    - lzma                     : no
    - zstd                     : no
    - iconv (libc)             : no
    - iconv (separate)         : no
    - icu                      : no
    - icu (lib64)              : no
    - native-atomic-int32-supported : no
    - message-compiler         : yes
    - native-syslog-supported  : no
    - pthread-supports-robust-mutexes : no
    - compiler-supports-ssse3  : no
    - compiler-supports-avx2   : no
    - gcc visibility           : no
*    - long double support      : no*

how  dose *long double support  determined*

is there a test file to *determine the future?*
*i just found *has_long_double_support.cpp
#include <boost/math/tools/config.hpp>

#ifdef BOOST_MATH_NO_LONG_DOUBLE_MATH_FUNCTIONS
#error "long double support is not supported by Boost.Math on this Plaform:
the long double version of the TR1 library will not be built."
#endif