Add files generated by "make depend", PGO and gcov to .gitignore

[Peter Davies]

> It shouldn't. What conf* files do you see?

conf23880
conf23880.dir
conf23880.file

Where "conf23880.dir" is an empty directory, "conf238800.file" is a
file containing only a newline and "conf23880" symlinks to
"conf23880.file".

Having looked into it, it seems that these files are usually deleted
by 'make depend' and are only temp files (I probably stopped "make
depend" with ^C to generate them).

The pattern is broken anyway since it also catches "configure", etc.

Peter