How to include math.h on a PIC?
The approach to low-memory coding is to not use math.h, and instead use INTEGERS, lookup tables, interpolation, and maybe Taylor Series (or other) expansions.  Figure out how much error you can tolerate (and how that error accumulates), and design your code to that.

Nice trig approximation review at http://www.ganssle.com/articles/atrig.htm