Build: avoid win32-incompatible functions

Dating from commit e1e0848625, I have been experimenting compiling errors (under win32).

    c:\users\geoffroy\documents\github\matplotlib\src\path_converters.h(895) : error C3861: 'seed48': identifier not found
            c:\users\geoffroy\documents\github\matplotlib\src\path_converters.h(893) : while compiling class template member function 'void Sketch<VertexSource>::rewind(unsigned int)'
            with
            [
                VertexSource=curve_t
            ]
            src/_path.cpp(1516) : see reference to class template instantiation 'Sketch<VertexSource>' being compiled
            with
            [
                VertexSource=curve_t
            ]

They seem to be linked with the use of traditional UNIX functions like `drand48` in *path_converters.h*. `seed48` and `M_PI` are also problematic.

Enclosed is a patch attempt which uses ANSI C function `rand` and `srand` instead, and at least compile correctly - with win SDK. (The code being quite complex I can't be sure with my limited C++ knowledge this is the best way to deal with it ; hope it can help though)