[Osi] compiling on macOS with gcc 8

[Antonio Frangioni]

> gcc 8 works for me.
> That -V is not known might not be the reasons that configure stopped.

That's completely right, my mistake. I stopped when I saw that, but in fact the error was

ld: library not found for -lSystem
collect2: error: ld returned 1 exit status

By this

https://github.com/Homebrew/legacy-homebrew/issues/45011

I learnt this was an Homebrew issue, and by

brew reinstall gcc

I was able to configure, and then make, the whole FlopC++ thing (hence, OSI) using gcc-8.

Well, almost: one has to use OSI 0.107.9 and Cbc 2.9.9, which is not what is fetched by FlopC++. Also, I was still getting an error from FlopC++:

../.././../src/Coin-OR/coin-FlopCpp/FlopCpp/src/MP_set.hpp:212:9: error: no matching conversion for functional-style cast from
      'const flopc::SUBSETREF' to 'flopc::MP_domain'
 return MP_domain(s);

Thanks to Enrico Gorgone, I now know that this can be solved by changing line 211 of ../FlopCpp/src/MP_set.hpp from

   MP_domain& operator()(const SUBSETREF& s) {
	return MP_domain(s);
    }

in

  const MP_domain& operator()(const SUBSETREF& s) {
	return MP_domain(s);
    }

Sorry for not spotting my own Homebrew issue. Hope this will still be useful to somebody.

	Best

	Antonio