build: use libc++ with clang on Travis

[richardlau]

Testing alternative fix for #23778.

Checklist

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[nodejs-github-bot]

@richardlau build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/1320/pipeline

[richardlau]

Did not appear to work 😞

[richardlau]

Trying more things.

[refack]

Did you try upgrading stdlibc++ to 4.9?

[richardlau]

@refack I'm trying to see if we can actually use clang (as we're attempting to). If we have to install packages then #23778 already covers this.

[refack]

I'm trying to see if we can actually use clang (as we're attempting to). If we have to install packages then #23778 already covers this.

From #23778 (comment) it seems that it does use clang, only it links against the GCC libstdc++-4.8, which has a bug.
(Run make with V=1 to see which compiler is actually used)

[refack]

There's -stdlib=libc++ that supposedly tell clang to use it's own libc++

[refack]

It works!

[richardlau]

It works!

😮. Going to sleep now, but next steps will be to see if all the environment variable changes were necessary. Maybe also -stdlib=libc++ should be set in CXXFLAGS instead of CXX.

[richardlau]

Travis overrides the CC/CXX env vars set under the env: key in the matrix:

[richardlau]

This PR allows Travis to build with clang without installing any other packages by making it use libc++ instead of libstdc++. Although it works I'm closing this as this is a non-standard way of building (we only normally use libc++ on macOS) and #23778 better replicates the supported toolchains (gcc 4.9.4 or later) in https://github.com/nodejs/node/blob/master/BUILDING.md#supported-toolchains.