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ARROW-2720: [C++] Defer setting of -std=c++11 compiler option to CMAKE_CXX_STANDARD, use CMake option for -fPIC

Small cleanup, since we were redundantly adding `-std=c++11` to the CXXFLAGS.

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

Closes #2995 from wesm/ARROW-2720 and squashes the following commits:

b7f713f3a <Wes McKinney> Set CMAKE_CXX_STANDARD in SetupCxxFlags.cmake so it is also used in Python build
e8bdefc48 <Wes McKinney> Defer setting of -std compiler option to CMAKE_CXX_STANDARD CMake option. Set CMAKE_POSITION_INDEPENDENT_CODE