Adding a parameter for use in toolchain 'using' line

Hi Build Experts,
I'm trying to update tools/build/src/tools/clang-vxworks.jam for newer version of the compiler.
Wind River's compiler team have decided to use a completely unmodified compiler, so the cross-build linker configuration is more complex.

I would like to propagate the build configuration from the project.config or user.config file so the complete configuration can be supplied in a single file for each target board/processor targeted.

With something like this:

using clang-vxworks : : $(cxx) :
                    <cflags>$(cflags)
                    <cxxflags>$(cxxflags)
                    <compileflags>$(compileflags)
                    <archiver>$(ar)
                    <linker>$(linker)
                    <library-path>$(library-path)
                    <libraries>$(libraries)
                    <crt0-obj>$(crt0-obj)
                    <linkflags>$(linkflags)
                    <linkflags-static>$(linkflags-static)
                    <linkflags-dynamic>$(linkflags-dynamic)
                    <linkflags-dll>$(linkflags-dll) ;

But some options on the "using" line appear to be regular "features" and some don't appear to be defined anywhere  as a formal feature?  Where are these "special features" declared, and how is there behavior different from a regular "feature"?
Many Thanks,

Brian
Wind River

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