Workaround LLVM/Clang 11.0 for Darwin builds (#2200)
Workaround LLVM/Clang 11.0 for Darwin builds (#2200)

LLVM/Clang 11.0 adds a `-Wundef-prefix=TARGET_OS_` build flag which
breaks this build flag.

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>
prometheus/node_exporter,prometheus/node_exporter,prometheus/node_exporter