libcxxwrap_julia for Julia 1.5: rebuild with GCC 8

[fingolfin]

This may fix the crash reported here: JuliaInterop/libcxxwrap-julia#75.

Once the binaries are built, I'll verify this locally with an artifact override.

CC @barche @benlorenz

[fingolfin]

I tested this on a Linux machine, and can confirm: with the current 0.8.5 build of libcxxwrap_julia_jll it crashes in Pkg.test("CxxWrap") which goes away if I use the artifact produced by this PR!

[benlorenz]

Thanks for testing. Do we need to build all dependent packages with gcc 8 as well when this is merged?

[barche]

Since libcxxwrap-julia is mostly header-based, building the dependents with gcc8 is probably recommended.

[fingolfin]

Of course right now we don't know whether this is a fix (i.e.: there is a compiler bug in GCC 7 which caused the original issue)l or a workaround (GCC 8+ optimize differently and hence hide the underlying bug). IMHO the tests @benlorenz performed give some plausibility to the former scenario; alas, that's no proof.

In the meantime, I suggest we merge this, and add preferred_gcc_version=v"8" or higher to all packages using libcxxwrap

[fingolfin]

@giordano please merge?

[giordano]

Did you test only on macOS or also on Linux? Edit: also on Linux

[fingolfin]

On both -- specifically, the crash only occurred on Linux so far, so I made that my priority.

[giordano]

I was worried about the possible symbols with a too high GLIBCXX version (official Julia binaries are built with GCC 7 and that's the libstdc++ currently shipped with Julia), but

% nm libcxxwrap_julia.so|c++filt|grep -Eo "GLIBCXX_[0-9.]+"|sort -u
GLIBCXX_3.4
GLIBCXX_3.4.11
GLIBCXX_3.4.20
GLIBCXX_3.4.21
GLIBCXX_3.4.9

shows 3.4.21 at most, which corresponds to GCC v5.1. Looks all good