Build libdmtx.a instead of libDmtx_Library.a for consistency with shared lib

This makes it easier to read by using the same name as the packaged version of the dmtx library on Linux.

Any other project which references the dmtx library will also need to be tweaked the same way (e.g. using -ldmtx instead of -lDmtx_Library)

For other projects:

```
find . -name *.pro -exec sed -i -e 's/-Dmtx_Library/dmtx/g'
```

may be sufficient to correct all references to the library.