using toolset vs toolset=

Dear Boost.Build community,

I have an issue when running b2 on Mac OS Sierra (10.12.6).

The project builds fine with command:
b2 toolset=clang sample

However, when I put
using clang ;
into ~/site-config.jam file
I'm getting following error:

WARNING: Unable to construct sample/sample of type EXE with these properties:
<asynch-exceptions>off <debug-symbols>on <exception-handling>on
<extern-c-nothrow>off <hardcode-dll-paths>true <host-os>darwin <inlining>off
<install-dependencies>off <link>shared <optimization>off <os>MACOSX <pch>on
<profiling>off <rtti>on <runtime-debugging>on <runtime-link>shared
<stdlib>native
<strip>off <suppress-import-lib>false <symlink-location>project-relative
<target-os>darwin <threading>single <toolset>clang <user-interface>console
<variant>debug <vectorize>off <warnings-as-errors>off <warnings>on
WARNING: Considered these as possible generators:
error: no generators were found for type 'EXE'
error: and the requested properties
error: make sure you've configured the needed tools

--debug-generators did not provide insights to me yet.

Do you have an idea what causes the error and how to fix it?

Regards
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