ninja/mac: Warn on conditional keys.
ninja/mac: Warn on conditional keys.

Now that ARCHS is supported, someone might try to use FOO[arch=i386], that
shouldn't fail silently.

BUG=chromium:122592,chromium:90185
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/22846002

git-svn-id: e7e1075985beda50ea81ac4472467b4f6e91fc78@1693 78cadc50-ecff-11dd-a971-7dbc132099af

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