Add configure flag --with-libunistring-prefix

[rockdaboot]

Fixes #214

@TurtleWilly @rb07 Can you test this please?

It looks like it works for me:

$ ./configure --enable-runtime=libidn2 --with-libunistring-prefix=/tmp
...
$ make
...
$ ldd tests/test-is-public
        linux-vdso.so.1 (0x00007ffd4179a000)
        libpsl.so.5 => /home/tim/src/libpsl/src/.libs/libpsl.so.5 (0x00007f2440e2c000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f2440c2d000)
        libunistring.so.5 => /tmp/lib/libunistring.so.5 (0x00007f2440a70000)
        libidn2.so.0 => /lib/x86_64-linux-gnu/libidn2.so.0 (0x00007f2440a3d000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f2440e46000)

[rb07]

Yes, it works for me.