When using C++20, optionally add in comma for varargs

[rm5248]

See: #344

The FMT macros now work with only one argument when using C++20 or greater.

[swebb2066]

You can now also undo the changes in throughput

[rm5248]

Unfortunately since this is a C++20 only fix, that build would fail with earlier versions.

The optional GCC fix as well (##__VA_ARGS__) only works if GNU extensions are enabled. So it works if you compile with -std=gnu++11 but fails with -std=c++11. There doesn't seem to be a way to test if this is enabled or not at compile time.