Add texinfo build target for document

Hi, I am trying to build numpy document in info format.  With this patch applied, I can build numpy.texi but compiling numpy.info from that file fails.  Here is the end part of the log.  There were a lot of warnings when building it which I omit here.

```
numpy.texi:196193: Node `Python Module Index' previously defined at line 196161.
/.../numpy/doc/build/texinfo//numpy.texi:196161: Prev field of node `Python Module Index' not pointed to.
/.../numpy/doc/build/texinfo//numpy.texi:196161: This node (Python Module Index) has the bad Next.
/.../numpy/doc/build/texinfo//numpy.texi:195560: Next field of node `Glossary' not pointed to (perhaps incorrect sectioning?).
/.../numpy/doc/build/texinfo//numpy.texi:196161: This node (Python Module Index) has the bad Prev.
makeinfo: Removing output file `numpy.info' due to errors; use --force to preserve.
make[1]: *** [numpy.info] Error 1
make[1]: Leaving directory `/.../numpy/doc/build/texinfo'
make: *** [info] Error 2
```

Any idea of how to fix this?

Note that I can still get (broken) info file if I run `makeinfo --no-split --force -o numpy.info numpy.texi`.  Should I add this to Makefile, instead of the line `make -C build/texinfo info`?