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ARROW-219: Preserve CMAKE_CXX_FLAGS, fix compiler warnings

Some undesired compiler warnings had crept into our build; future warnings should fail the build now.

Author: Wes McKinney <wesm@apache.org>

Closes #92 from wesm/ARROW-219 and squashes the following commits:

fd68a74 [Wes McKinney] Buglet
6507351 [Wes McKinney] Fix clang warning
0f9e3ca [Wes McKinney] Preserve CMAKE_CXX_FLAGS, fix compiler warnings