`highs-sys` crate fails to build with gcc v15

[alexdewar]

MUSE 2.0 currently won't build on my machine since my distro upgraded to gcc v15. I've opened a PR upstream. Hopefully the fix will find its way to the highs-sys crate before long.

[alexdewar]

Workaround: if you have an older version of gcc installed alongside gcc v15, you can use that to compile instead, e.g.:

env CXX=g++-14 cargo build

[alexdewar]

Adding this to the v0.2.0 milestone because it would be useful to be able to compile old versions of MUSE2 in future, as far as is possible