[clang-win, mingw] Support of automatic tss cleanup for native threading API not available

From a new CI job targeting `toolset=clang-win`:
(warning: large data link: https://ci.appveyor.com/project/jeking3/thread/builds/20524620/job/svesllxvmphivmuq?fullLog=true#80760)
```
testing.capture-output bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.run
====== BEGIN OUTPUT ======
Running 7 test cases...
tss_instances = 0; tss_total = 5
tss_instances = 5; tss_total = 5
libs/thread/test/test_tss.cpp(208): error: in "test_tss": Support of automatic tss cleanup for native threading API not available
000001CF8AB26D70
*** 1 failure is detected in the test module "Boost.Threads: tss test suite"

EXIT STATUS: 201
====== END OUTPUT ======

set status=0
if %status% NEQ 0 (
echo Skipping test execution due to testing.execute=off
exit 0
)
"bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.exe" > "bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.output" 2>&1
set status=%ERRORLEVEL%
echo. >> "bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.output"
echo EXIT STATUS: %status% >> "bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.output"
if %status% EQU 0 (
copy "bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.output" "bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.run"
)
set verbose=0
if %status% NEQ 0 (
set verbose=1
)
if %verbose% EQU 1 (
echo ====== BEGIN OUTPUT ======
type "bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.output"
echo ====== END OUTPUT ======
)
exit %status%
...failed testing.capture-output bin.v2\libs\thread\test\test_tss_lib.test\clng-win-7.0.0\rls\adrs-mdl-64\cxstd-11-iso\thrdp-wn32\thrd-mlt\test_tss_lib.run...
```