[0.26] Missing libiwyu.so.22.1 library

The `include-what-you-use` binary builds and links with `libiwyu.so.22.1`, but the CMake doesn't install it, resulting in errors when attempting to run the installed package:

```
can't install iwyu:
  - nothing provides libiwyu.so.22.1()(64bit) needed by iwyu-0.26-1.fc45.x86_64
```

The regression introduced by https://github.com/include-what-you-use/include-what-you-use/commit/9c7a6a3c08af3ecfc6e0a8716455435c696b7bb2 and https://github.com/include-what-you-use/include-what-you-use/commit/e9a08a9e6cb32eff8120473ccc51e035d5942451 commits.

Full ldd output:
```
$  ldd ./include-what-you-use
        linux-vdso.so.1 (0x00007f511846c000)
        libiwyu.so.22.1 => not found
        libLLVM.so.22.1 => not found
        libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007f5118000000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f5117e0d000)
        libm.so.6 => /lib64/libm.so.6 (0x00007f511834e000)
        libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f5118320000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f511846e000)
```

> We didn't plan to install these libraries, they are only for internal use.

Then these libraries need to be `STATIC` or `OBJECT` to prevent linking against them dynamically.

UPD. Fedora build system always explicitly sets the `-DBUILD_SHARED_LIBS:BOOL=ON` build flag. This may cause `BUILDTREE_ONLY` to be treated as `SHARED`.
UPD2. Explicitly setting the `-DBUILD_SHARED_LIBS:BOOL=OFF` can be used as a workaround. Most Linux distributions will need it.

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