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ARROW-3837: [C++] Add GFLAGS_IS_A_DLL define to fix Windows build

I'm not sure why this only broke now with gflags 2.2.2, but the default value for this define is generated by the gflags build system, and may be 1 or 0. Setting it to 0 indicates that it's our intent to statically link, and so the `__declspec(dllimport)` annotations won't be enabled

Author: Wes McKinney <wesm+git@apache.org>

Closes #3000 from wesm/ARROW-3837 and squashes the following commits:

d172fe2da <Wes McKinney> Set GFLAGS_IS_A_DLL define to restore ability to statically link using conda-forge gflags 2.2.2