[C++] Clean up cmake CXX_STANDARD and PIC flag setting

[asfimport]

We're using -std=c++11 in a few non-external project places as well as setting -fPIC. CMake provides the CMAKE_CXX_STANDARD flag (which we are also using) and the CMAKE_POSITION_INDEPENDENT_CODE flag for setting these options in a cross platform way (where it matters).

We should use these flags instead of using platform conditional checks to set their values explicitly.

Reporter: Phillip Cloud / @cpcloud
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #2995

_{Note: This issue was originally created as ARROW-2720. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Definitely would be good to use the CMAKE_* stuff where we have control over it. We'll still have to pass -fPIC to some of the external projects, but we could handle that in ThirdpartyToolchain.cmake

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2995
#2995