remove c-side unicode characters

[goulart-paul]

Remove unicode characters from the C / C++ side of the interface. Addresses #27.

[CLAassistant]

All committers have signed the CLA.

[jwnimmer-tri]

I've double-checked that these three μ are the only non-ASCII characters is the code. I think this is OK to merge.

FYI There are non-ASCII in the comments, but these do not cause any problems:

     * The first two parameters supplies a pointer to an array of nonnegative powers “alpha” of the 
     * The “alpha” terms must sum to 1.