[C++] Remove -Wconversion from build flags

[asfimport]

I would request removing -Wconversion from the CI build flags. It is overly finicky, generating errors for things such as converting a size_t to double. Furthermore, gbenchmark fails building with it, making it difficult to exercise locally.

I will mention that MSVC's conversion warnings are vastly more reasonable.

Reporter: Antoine Pitrou / @pitrou

_{Note: This issue was originally created as ARROW-3522. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
We shouldn't be passing -Wconversion into EP builds, are you passing -DARROW_CXXFLAGS="-Wconversion" or something else?

[asfimport]

Antoine Pitrou / @pitrou:
Yes, that's what I'm doing. Is there another way?

[asfimport]

Wes McKinney / @wesm:
Is the problem with gbenchmark happening when individual benchmarks are being built or when building the gbenchmark library itself?

[asfimport]

Antoine Pitrou / @pitrou:
When building gbenchmark itself.

[asfimport]

Wes McKinney / @wesm:
OK, how is -Wconversion getting passed to the gbenchmark build? The flags passed to ARROW_CXXFLAGS are not supposed to be propagated into the external projects

[asfimport]

Antoine Pitrou / @pitrou:
Well, there is:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${ARROW_CXXFLAGS}")

Then:

set(EP_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${UPPERCASE_BUILD_TYPE}}")

Then:

set(GBENCHMARK_CMAKE_CXX_FLAGS "-fPIC -std=c++11 ${EP_CXX_FLAGS}")

So it seems they will?

[asfimport]

Wes McKinney / @wesm:
That first line is called after include(ThirdpartyToolchain), though

https://github.com/apache/arrow/blob/master/cpp/CMakeLists.txt#L383

In any case we should get to the bottom of the issue you're having

[asfimport]

Antoine Pitrou / @pitrou:
Ah! It turns out an environment variable named CXXFLAGS that I defined in my scripts was taken up by CMake :-/