ANSI C Compliance

[wojciech-graj]

Hey, what are your thoughts on making the C library ANSI (C90) compliant?

The following changes would be required:

• Replace C++-style comments (//) with C-style comments (/**/)
• Move declarations to the start of code blocks (already occurs in most of the code)
• Replace %lf format specifier with %f in printf in TestSGP4.c

I'd be happy and make these changes, so feel free to assign the issue to me.