[#113] Define M_PI when not already defined
[#113] Define M_PI when not already defined

In certain environments, the M_PI constant is not defined in the
math.h header of the C standard library. From what I read it looks
like M_PI was omitted past the C99 standard. In any case, a fix
is introduced where the constant is explicitly defined.

Fixes #113.

abelfunctions/abelfunctions,cswiercz/abelfunctions,cswiercz/abelfunctions,abelfunctions/abelfunctions,abelfunctions/abelfunctions,cswiercz/abelfunctions