[Nix-dev] sigil: new package

[Michael Raskin]

Committed with some cleanup. 

Notes for the future.

0. Please make sure that your mail program doesn't mangle the patch! 
One of the ways to ensure it is to attach patch file.

1. Use version, not the_version for version number

2. Please remove commented-out shell code or explain why leave it in the
expression

3. Usually it is a good idea to define pre* and post* additions instead
of overriding the entire build. For example, you could probably define
postUnpack to do the fixes and set cmakeFlags.