Switch to C89 comments in prep for making the whole codebase C89 compatible.

This should be whitespace/comment only changes and should produce the same
object code.
Correct .gitignore to correctly account for build-aux.
Merge pull request #106

8ca6a9c Correct .gitignore to correctly account for build-aux. (Phillip Mienk)
Merge pull request #105

71712b2 Switch to C89 comments in prep for making the whole codebase C89 compatible. (Gregory Maxwell)
Avoid division when decomposing scalars

- In secp256k1_gej_split_exp, there are two divisions used. Since the denominator is a constant known at compile-time, each can be replaced by a multiplication followed by a right-shift (and rounding).
- Add the constants g1, g2 for this purpose and rewrite secp256k1_gej_split_exp accordingly.
- Remove secp256k1_num_div since no longer used