fix #251: adjust ostringstream constructor for recent libstdc++

[benlorenz]

Better fix this time, just 4 characters. As it turns out, only the default constructor for ostringstream requires GLIBCXX_3.4.26, we can just use an empty string instead.

---- obsolete first try:
@fingolfin @kalmarek

Unfortunately just dlopening a new libstdc++.so.6.0.26 doesn't help because the DT_NEEDED of libpolymake.so is just libstdc++.so.6 which will still resolve to the old one from julia which is also in memory.

But since we love having more and more dependencies we can patch the library with patchelf to use the full filename of the correct libstdc++. I tried looking for options to gcc to do something similar at link time but that seems impossible.

PS: all of the above applies only to Sys.islinux()

[benlorenz]

edit: please ignore

Unfortunately adding this new libstdc++ broke exactly the show method mentioned in #251

Polymake.Vector{Int64}: Test Failed at /home/runner/work/Polymake.jl/Polymake.jl/test/vectors.jl:88
  Expression: string(V) == "pm::Vector<long>\n5 10 3"
   Evaluated: "pm::Vector<long>\n" == "pm::Vector<long>\n5 10 3"

[fingolfin]

nice!

[kalmarek]

@benlorenz I tried this and see no adverse effects:

• strings libpolymake.so | fgrep GLIBCXX_3.4.26 | c++filt is empty
• Polymake.Vector([1,2,3]) prints correctly (and all the tests pass).

[benlorenz]

The print bug was with my previous try where I pulled in the new libstdc++.

I added a version bump so we can directly make a new release if needed, feel free to merge this as soon as the tests are done.