Make C extension optional
Make C extension optional

C excension is just a speedup, so we don't need to fail the installation
if the build is failed.

It's a follow-up fix for 6ec33af0d01fe73d5ce2c48a9d784fb47596d641.

Closes: issue #393

Kronuz/pyScss,Kronuz/pyScss,Kronuz/pyScss,Kronuz/pyScss