ARROW-3659: [CI] Fix Travis matrix entry 2 documentation to use gcc

[pcmoritz]

This is a little subtle and is needed because of the order in which Travis evaluates the command line variable definitions (see the JIRA issue).

I also needed to remove a warning due to this: cython/cython#2269
We might be able to bring the warning back once we can depend on cython with cython/cython#2669

[pcmoritz]

Can somebody help looking into what is going on with the code coverage? I'm not sure how it is supposed to work.

[wesm]

This is a known issue, we were trying to get clang to work with the coverage in my recent PR but didn't figure it out

[wesm]

I'm AFK today but @pitrou may have an idea

[pcmoritz]

Ah ok, thanks!

[pitrou]

I don't think we care much about the compiler here, so I'd just rename the CI entry to reflect that it uses gcc, not clang (there are other entries using clang anyway). What do you think?

[pcmoritz]

Ok, I replaced it with gcc (4.9 actually to make it compatible with gandiva for #2822) and it should be ready to merge.

[pitrou]

Why MATRIX_EVAL?

[pcmoritz]

because CC and CXX get exported before Travis re-exports them according to the build matrix (see the JIRA issue printout).

[pcmoritz]

(the other build matrix entries do it the same way)

[pitrou]

Uh, I see, thanks.

[kou]

FYI: We can find the MATRIX_EVAL technique in Travis CI document: https://docs.travis-ci.com/user/languages/c/#gcc-on-linux

[pitrou]

+1

[codecov-io]

Codecov Report

Merging #2878 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2878      +/-   ##
==========================================
- Coverage   87.63%   87.55%   -0.09%     
==========================================
  Files         412      412              
  Lines       64048    64048              
==========================================
- Hits        56130    56076      -54     
- Misses       7846     7898      +52     
- Partials       72       74       +2

Impacted Files	Coverage Δ
go/arrow/math/int64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/memory/memory_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/float64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/uint64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/float64_amd64.go	33.33% <0%> (-33.34%)	⬇️
go/arrow/math/int64_amd64.go	33.33% <0%> (-33.34%)	⬇️
go/arrow/math/uint64_amd64.go	33.33% <0%> (-33.34%)	⬇️
go/arrow/math/math_amd64.go	31.57% <0%> (-31.58%)	⬇️
go/arrow/memory/memory_amd64.go	28.57% <0%> (-28.58%)	⬇️
cpp/src/arrow/compute/kernels/cast.cc	91.27% <0%> (-0.04%)	⬇️
... and 3 more

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[kou]

CI is green.
I'll merge this.