I think it's because clang doesn't know about -mthreads.
Does clang/Windows using the MingW/gcc RTL support multithreading ? If so is
there a command line option for this ?
IIRC, not yet.
OK, Thanks ! That does not help with Boost testing as quite a few Boost libraries either need multi-threading or test for multi-threading as part of their overall tests.
neither the clang build with MingW/gcc RTL nor the clang-cl with VC++ RTL is really ready yet. For other more simplified uses of clang on Windows outside of Boost either may be fine.
I realize that clang developers have different priorities so I will revisit clang on Windows sometime in the future to see if some of the issues I have encountered have been resolved.