Choosing between libc++ or libstdc++ based third party libraries during linking

[Clemens Lang]

Hi,

> The ECW support in GDAL (#43678) is tied to the use of an external dylib (non
> open-sourced) that comes in two versions: one is linked against the old
> libstdc++, the other against libc++. Question is: how to decide in the
> Portfile which one to use?

Switch depending of the value configure.cxx_stdlib. It's either libstdc++ or
libc++.

-- 
Clemens Lang