Avoid masking math.h exported variables

Avoid masking math.h exported variables

Our beloved math.h exports, from bits/mathcalls.h, a bare "y1" symbol.
Apparently, it's unthinkable for code including <math.h> to also declare
arguments or variable named "y0", "y1" and "yn".

Anyway, the quick fix already used elsewhere in Clutter's codebase is
to rename the colliding variables "y_0", "y_1" and "y_n" - and
obviously everything similar to them as well, using the same pattern.