GitLab

Motivation:

    I have downloaded a barebones Linux system built for 64-bit ARM
    from linaro.org and a ARMv8 Foundation Model from arm.com to run
    it on.  This will let us build and test Skia on ARM64 before we
    aquire hardware to allow that.  This CL introduces the changes to
    the build files necessary to build Skia on a barebones embedded
    Linux system.  I tested it with the aarch64 GCC compiler provided
    by linaro.org.

Changes:

    Add a "barelinux" target_os for the DEPS file.  Add an optional
    git download of zlib.

    Changes to gyp files: these changes abstract out libpng, libz, and
    giflib so that images.gyp doesn't know whether they are static or
    dynamically linked.  I also add the variables skia_giflib_static,
    skia_libpng_static, skia_zlib_static, and skia_freetype_static,
    all of which default to false but when set to true will override
    the behavior of the giflib, libpng, zlib, and freetype build
    t...