ARROW-6457: [C++] Always set CMAKE_BUILD_TYPE if it is not defined
ARROW-6457: [C++] Always set CMAKE_BUILD_TYPE if it is not defined

This may be a dup of another issue, but I encountered this locally and this produces default RELEASE build type with both Ninja and MSVC build generator for me

Closes #5282 from wesm/ARROW-6457 and squashes the following commits:

aad180da9 <Wes McKinney> Always set CMAKE_BUILD_TYPE if it is not defined
9da306a27 <Wes McKinney> Always set CMAKE_BUILD_TYPE if it is not defined

Authored-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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