[BUG] Cannot link with :gcc built-in toolchain on MacOS

[tcbrindle]

Describe the bug

In MacOS, /usr/bin/g++ actually invokes clang++. A simple test project using dds build -t :gcc compiles successfully, but fails to link as it cannot find libstdc++fs (and even if it could it probably wouldn't work).

Steps to Reproduce

1. Write a simple test project
2. Attempt to build with dds build -t :gcc
3. Receive linker error message

dds Output

22:35:15] [info ] [example] Compile: example.test.cpp
[22:35:16] [info ] [example] Compile: example.test.cpp                          -     757ms [1/1]
[22:35:16] [info ] Compilation completed in 801ms
[22:35:16] [info ] Archiving completed in 0ms
[22:35:16] [info ] [example] Link: test/example                  
[22:35:16] [info ] [example] Link: test/example                   -     86ms
[22:35:16] [error] Failed to link executable [/Users/tristan/Coding/dds-experiments/example/_build/test/./example]. Link command was [g++ -fPIC -fdiagnostics-color /Users/tristan/Coding/dds-experiments/example/_build/obj/example.test.cpp.o /Users/tristan/Coding/dds-experiments/example/_build/_test-driver/v1/catch2/catch-main.cpp.o -pthread -lstdc++fs -o/Users/tristan/Coding/dds-experiments/example/_build/test/./example] [Exited 1], produced output:
ld: library not found for -lstdc++fs
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[22:35:16] [error] Linking a runtime binary (executable/shared library/DLL) failed
[22:35:16] [error] 
Linking a runtime binary file failed. There are a variety of possible causes
for this error. Refer to the documentation for more information.

[22:35:16] [error] Refer: https://vector-of-bool.github.io/docs/dds/err/link-failure.html

Desktop (please complete the following information):

• Operating System: MacOS
• Compiler: Clang pretending to be GCC
• dds Version: 0.1.0-alpha.2

Additional context

Perhaps on MacOS, the built-in :gcc toolchain should be special-cased to use the Clang Compiler-ID setting, rather than GNU? (Which also doesn't work right now, but that's #11 )

[vector-of-bool]

Urgh. This is one of those very aggravating platform quirks.

In this case, I think things will be fixed by removing -lstdc++fs as a default link flag. This was a flag that I needed for my own case but should never have been applied universally. Other than that, gcc being aliased to AppleClang should "just work."