The goal is to do the equivalent of clobbering memory, which forces
the compiler to keep side effects that are external to the benchmarked
function, while allowing it to optimize inside the benchmarked function.

E.g. this cannot be optimized away:
```cpp
BENCHMARK("foo") {
    global_count += 1;
};
```

while this can:
```cpp
BENCHMARK("bar") {
    size_t local_count = 0;
    for (size_t i = 0; i < 100; ++i) {
        local_count += 1;
    }
};
```

Otherwise intel c++ 19.1 cause an error
"expression preceding parentheses of apparent call must have (pointer-to-) function type CATCH2"

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

This improves the SelfTest build times by about 3% (measured
with Clang 10 on random Linux box I had lying around).

This should also be done for test files, but that has lower priority.

This commit also strips the old copyright comment header in touched
files, as those will also be replaced with a more standardized and
machine-friendly version.

Now that Catch2 is a proper library, we can always build the full
library (comparatively minor slowdown) and the user can avoid
including benchmarking headers to avoid the compilation slowdown.