This broke installation on Windows, because the suddenly started
with a '/' instead of a drive letter.

Closes #3019

Having the ability to configure the installation path at config
time, and the ability to change the prefix at install time is not
enough, apparently people also use env var to redirect them instead.

Closes #3006

Adding unreachable to `FAIL` and `SKIP` made Clang become very good
at figuring out that tests of `FAIL` and `SKIP` will never return
and thus could be marked as [[noreturn]].

To avoid introducing lot of warning suppression noise into the
test files, we just disable it instead.

This enables us to generate pkg-config with proper paths if the
installation prefix is decided at install time, e.g. with
```
cmake --install build-dir --prefix /path/to/somewhere
```

It also means that we can use CMake's generator expression to get
the _real_ final name of the libraries, e.g. including debug postfix.

Closes #2979

fix #2755

https://cmake.org/cmake/help/v3.16/module/FindPythonInterp.html

2 spaces seems to be the more common indentation level so that's what
I unified around.