UseVala.cmake: added argument SOURCE_DIRECTORY to vala_precompile

With this patch one can specify a SOURCE_DIRECTORY in vala_precompile.
Behaviour of DIRECTORY has not changed.

Instead of writing
```cmake
vala_precompile(C_SOURCES
    SOURCES
        path/to/code/file1.vala
        path/to/code/file2.vala
)
```
you can write
```cmake
vala_precompile(C_SOURCES
    SOURCES
        file1.vala
        file2.vala
    SOURCE_DIRECTORY
        path/to/code
)
```

The c-code is located in the same place in both examples