Unconditional linking with libstdc++

[alexey-lysiuk]

SLADE Version: 3.2.0 as of 29c9b9bd8142ae95e4ab387424d5056162f88827 Operating System: All

Issue Details: Regardless of the actual C++ Standard library implementation, SLADE attempts to link with libstdc++, and its filesystem implementation in particular. This is not quite correct even on Linux where libc++ is an acceptable alternative. On macOS with the default toolchain, this cannot work by definition.