BUILD: makefile: Redirect stderr to /dev/null when probing options

It is a workaround to avoid a clang 11 bug that exits with SIGABRT when
stderr is redirected to stdin. This bug was already reported few weeks ago:

  https://bugs.llvm.org/show_bug.cgi?id=49463

But because it is pretty annoying, the standard error is now redirected to
/dev/null.
1 file changed