Supply default PLATFORM_NAME
Supply default PLATFORM_NAME

When building with xcodebuild, PLATFORM_NAME is empty. This commit
just supplies a default of "macosx" in that case. It's probably not
the most elegant solution, but should fix the build.

This addresses the "can't find crate for `std`" errors reported in
issue #1.

google/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,modelorganism/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,google/xi-editor,google/xi-editor,modelorganism/xi-editor,modelorganism/xi-editor