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ARROW-3437: [C++] Use older API for boost::optional, remove gtest include from prod code, remove -static-libstdc++ flags

With these changes I can successfully compile Gandiva using stock Ubuntu 14.04 apt packages and gcc 4.8.5. I also found a stray use of a gtest include in production code

I removed the code about `-static-libstdc++`. The issue here is that if you're going to use -static-libstdc++ and -static-libgcc, this needs to be passed consistently for every link target. It would be incorrect for libgandiva.so to be linked that way, but libarrow.so to be not. So I think the correct approach here to pass this in via CMAKE_SHARED_LINKER_FLAGS.

Also resolves ARROW-3436, ARROW-3501

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

Closes #2998 from wesm/ARROW-3437 and squashes the following commits:

ba7e8c74 <Wes McKinney> Use older API for boost::optional, remove gtest include, remove -static-libstdc++