Libraries install to bin/ when CMAKE_INSTALL_BINDIR is set

In `src/CMakeLists.txt`, the LIBRARY and ARCHIVE destinations use `CMAKE_INSTALL_BINDIR`:

```cmake
install(TARGETS srcsax_shared srcsax_static
        RUNTIME DESTINATION bin
        LIBRARY DESTINATION ${CMAKE_INSTALL_BINDIR}
        ARCHIVE DESTINATION ${CMAKE_INSTALL_BINDIR}
       )
```

Also, include `GNUInstallDirs` before `src` is processed.
