Adds .gitignore rules for files generated by autoreconf

When running `autoreconf -fvi`, several files were generated
that were not ignored by the existing .gitignore rules:
```
aclocal.m4 configure m4/libtool.m4 m4/ltoptions.m4 m4/ltsugar.m4
m4/ltversion.m4 m4/lt~obsolete.m4
```

All should now be ignored.