This define is only needed when linking against Chromiums libc++ which we currently
do not ship / expose the symbols of. We probably should make those symbols visible and
actually ensure that electron-rebuild et. al link against our libc++ instead of the system libc++
but for now this fixes compilation issues on macOS where the default system clang links to the system libc++
which does not (obviously) use the Chromium ABI namespace.
For our nan tests which do link against Chromiums libc++ we define the ABI namespace in the spec runner.
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