opensp fails to build ==> fix

[Ken Cunningham]

I was not a fan of defaulting 10.6.8 to clang 9.0. After all the years of special-casing 10.6.8, I was hoping for some quiet times, and defaulting to clang 5.0, or maybe 6.0, would have provided that.

I think we're the canary in the coal mine right now.

For compensation, all macports devs should be required to set their default compiler to clang-9.0 :). We might as well have some company !

K