Syntax correction, little optimization, and dependency arranged.

Hi, I modified the code to use it for my project as the following...
* using QZSS functions only when ENAQZS is defined.
* const qualifier correction due to error generated by my C++ compiler.
* index correction because "nav->eph[0].sat!=nav->eph[0].sat||nav->eph[0].iode!=nav->eph[0].iode" is always false.

Thanks,