Use pkg-config to locate external HEALPix

The `build_healpix` subcommand morphs into `build_external_clib`,
which now supports locating any externally installed package with
`pkg-config`, falling back to building from bundled source.
    
To add another `pkg-config` style package, add a new entry to the
`libraries` argument to `setup()`, that looks like this:
    
    ('lib_name', {
        'pkg_config_name': 'name_of_pc_file',
        'local_source': 'path_to_bundled_source_distribution'})
    
As an example:
    
    ...
    libraries=[
        ('cfitsio', {
            'pkg_config_name': 'cfitsio',
            'local_source': 'ext/cfitsio'})],
    ...