Default libprefix on Debian like distribution seems not good.

By default, on 64 bits Debian like distribution, meson will install library in `/usr/local/lib/x86_64-linux-gnu`.
However, this directory is not in the library searched path by default (`/usr/local/lib` is), and so, the library cannot be linked.

Here a small reproducer using this small test project [test_meson.tar.gz](https://github.com/mesonbuild/meson/files/1090937/test_meson.tar.gz) :

```
$ cd test_meson
$ meson src build
$ cd build
$ sudo ninja install
$ mytestprogram
mytestprogram: error while loading shared libraries: libmytestlib.so: cannot open shared object file: No such file or directory
```

To be honest, I'm not sure it is a `meson` bug. On Fedora, `/usr/local/lib` is even not in the default search paths. Searching a bit about this issue, it seems that it is not a easy case, different distributions will set different multiarch libprefix and default library search paths. (and `lib/x86_64-linux-gnu` is the good libprefix if installing in `/usr`)

However, it seems really surprising, that using the default options, meson is installing something "not working".

Start with the attached test_meson.tar.gz reproducer and run the shown Meson and ninja install commands on a 64-bit Debian-like system. Inspect the default library installation path and the resulting dynamic-library lookup behavior; done means the expected default installation behavior is established and the issue's outcome is covered by an appropriate regression or documented as distribution-specific.

Written by the indexing model from the issue text.