Disable maintainer mode when compiling healpix_cxx

`pip` does not preserve file modification times when unpacking tarballs.
This causes `make` to think that it needs to rerun `automake` when
building `healpix_cxx`. Since only developers generally have `automake`,
this causes the installation to fail.

This patch works around the deficiency in `pip` by adding
`--disable-maintainer-mode` to the flags passed to `configure`. As a
result, `make` will not attempt to re-run `automake`.

Note that we need to add

    AM_MAINTAINER_MODE([enable])

to `healpix_cxx`'s `configure.ac` file in order to make `configure`
recognize this option.

Fixes #155.