Support optional option

[messense]

Just like distutils:

https://docs.python.org/3/distutils/setupscript.html#other-options

The optional option is a boolean; if it is true, a build failure in the extension will not abort the build process, but instead simply not install the failing extension.

[fafhrd91]

what is your pypi username?

[messense]

@fafhrd91 It's messense

[fafhrd91]

I gave you permission for setuptools-rust. release process is manual at the moment

[messense]

Crap...

Collecting setuptools-rust
  Downloading setuptools-rust-0.6.2.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/var/folders/x5/5tc76rdj7yq_7cbc4g1mpn9c0000gn/T/pip-build-XocbqP/setuptools-rust/setup.py", line 15, in <module>
        (open('README.rst').read(), open('CHANGES.rst').read())),
    IOError: [Errno 2] No such file or directory: 'CHANGES.rst'

    ----------------------------------------

sdist didn't add CHANGES.rst... will need a new version...

[fafhrd91]

Ok, np

[messense]

Fixed. Committed MANIFEST.in

[messense]

https://github.com/PyO3/setuptools-rust/blob/master/setuptools_rust/build.py#L166

If Rust is not installed, optional extensions won't work because get_rust_version() throws an uncaught exception.