Build Expat on all platforms

* bump version to @android-6.0.1_r55 (2.1.1)

* add as a dependency to the 'xml' target (for upcoming http://crrev.com/2142893006)

* tweak build for Mac, Win:

  - define HAVE_EXPAT_CONFIG_H (same as in Android.mk)
  - use -Wno-missing-field-initializers (same as in Android.mk)
  - suppress MSVS warning 4244 (same as ..cmake/Utilities/cmexpat/expatConfig.h.in)

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Review-Url: https://codereview.chromium.org/2150603004
3 files changed