postgis: allow on Darwin

To get PostGIS going on Darwin:
1. Add libiconv, as is often required.
2. Expand platforms to `platforms.all`.
3. Deal with PostGIS' quirky build system.

PostGIS' configure.ac has the following gem:

  AC_MSG_RESULT([------------------------------------------------------------------------])
  AC_MSG_RESULT([  WARNING: You have set the --prefix to '$prefix'. But we mostly    ])
  AC_MSG_RESULT([  ignore the --prefix. For your info, using the values determined from ])
  AC_MSG_RESULT([  $PG_CONFIG we will be installing:   ])
  AC_MSG_RESULT([    * postgis shared library in $PGSQL_LIBDIR ])
  AC_MSG_RESULT([    * postgis SQL files in $PGSQL_SHAREDIR/contrib/postgis-$POSTGIS_MAJOR_VERSION.$POSTGIS_MINOR_VERSION ])
  AC_MSG_RESULT([    * postgis executables in $PGSQL_BINDIR ])
  AC_MSG_RESULT([------------------------------------------------------------------------])

This is suggestive of some assumptions in the build system, which are
revealed when building in Nix on Darwin: the build fails because the
postgres binary cannot be found in the install prefix specified for
postgis; vis.

  cc x -bundle_loader $POSTGIS_PREFIX/bin/postgres

This bundle_loader parameter is only available on Darwin, and this
problem doesn't appear to affect Linux systems.

The solution presented here is to symlink the postgres binary where
PostGIS expects it to be, and then remove it after the build completes.

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