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ARROW-1624: [C++] Fix build on LLVM 4.0, remove some clang warning suppressions

I'm going to quick make a pass through later today and see how many of these warning suppressions I can remove

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1148 from wesm/warning-fixes and squashes the following commits:

e930152a [Wes McKinney] Only build compute modules if -DARROW_COMPUTE=ON
d6ca7ac0 [Wes McKinney] Slight refactor of CMakeLists.txt to move Arrow library setup to src/arrow
e2c61a2e [Wes McKinney] Use -Wno-unknown-warning-option
3d2d7265 [Wes McKinney] Fix travis CI script
6d0d4117 [Wes McKinney] Use BUILD_WARNING_LEVEL in Travis CI
1bec4a76 [Wes McKinney] Fix some more compiler warnings
cae05fb5 [Wes McKinney] Fix documentation compiler warnings
f76b2b93 [Wes McKinney] Fix a bunch of documentation warnings
ac54e2df [Wes McKinney] Remove some clang warning suppressions, fix warnings
6935b8c8 [Wes McKinney] Fix compiler warnings with clang-4.0