This changes the installation of the libraries to have version numbered
symlinks on systems that support it. The install changes from:

```
installation/
└── lib
    ├── libh2o-evloop.so
    ├── libh2o.so
    └── pkgconfig
        ├── libh2o-evloop.pc
        └── libh2o.pc
```

Into:

```
installation/
└── lib
    ├── libh2o-evloop.so -> libh2o-evloop.so.0.9
    ├── libh2o-evloop.so.0.9 -> libh2o-evloop.so.0.9.22
    ├── libh2o-evloop.so.0.9.22
    ├── libh2o.so -> libh2o.so.0.9
    ├── libh2o.so.0.9 -> libh2o.so.0.9.22
    ├── libh2o.so.0.9.22
    └── pkgconfig
        ├── libh2o-evloop.pc
        └── libh2o.pc
```