Suppression of TBB warnings (c9d74075911) leads to missing dependency

When the built-in TBB is used is TBB_INCLUDE_DIRS is set to

${CMAKE_BINARY_DIR}/include

To suppress warning the syntax

include_directories(SYSTEM ${TBB_INCLUDE_DIRS})

This leads the directory where the ROOT headers are to be marked as "system".
On MacOS, this means that the line

-isystem include

is added to the compilation line.

When clang generates dependency (at least with the flag we are using), it does *NOT* include any system header in the list produced by -MF.
In this case this means that NO ROOT headers is included in the .d files. Leading of course to corrupt builds as soon as any of the header that should have been listed is modied,

See for example

ninja -t deps  tree/treeplayer/CMakeFiles/TreePlayer.dir/src/TFormLeafInfo.cxx.o

tree/treeplayer/CMakeFiles/TreePlayer.dir/src/TFormLeafInfo.cxx.o: #deps 1, deps mtime 1508358059 (VALID)

    /Users/pcanal/root_working/code/rootcling/tree/treeplayer/src/TFormLeafInfo.cxx

or

$ ninja -t deps core/imt/CMakeFiles/Imt.dir/src/TTaskGroup.cxx.o

core/imt/CMakeFiles/Imt.dir/src/TTaskGroup.cxx.o: #deps 1, deps mtime 1508351306 (VALID)

    /Users/pcanal/root_working/code/rootcling/core/imt/src/TTaskGroup.cxx

Cheers,
Philippe.