Clang on Cygwin64 compains about -mthreads

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Environment and version details

• Operating System+version: Windows 10, Cygwin64
• Compiler+version: Clang 8.0.1
• Shell: Bash
• B2 Version: Boost.Build develop, revision ae1b579

Brief problem description

Boost.Build inserts -mthreads argument to the compiler command line, which the compiler doesn't recognize.

Also, the compiler is detected as clang-linux-4, even though it is clang 8 on Cygwin. At least the version seems off.

Steps to reproduce the issue

1. Build b2 in Cygwin64. Use the workaround proposed in Building b2 on Cygwin64 fails #37.
2. Build Boost.Filesystem tests (I use it as an example; the problem likely reproduces with any project). Command line: ./bjam.exe -j 8 toolset=clang cxxstd=17 address-model=64 variant=release threading=multi runtime-link=shared optimization=speed inlining=full libs/filesystem/test

Actual behavior summary

The compiler output contains lots of warnings like these:

clang-linux.compile.c++.without-pch bin.v2/libs/filesystem/test/macro_default_test.test/clang-linux-4/release/cxxstd-17-iso/threading-multi/visibility-hidden/macro_default_test.obj
clang-8: warning: argument unused during compilation: '-mthreads' [-Wunused-command-line-argument]

Expected behavior summary

No such warnings should be present. The compiler version should be correct, instead of clang-linux-4.