Build issue under Cygwin64

[arpruss]

I get a whole bunch of errors like:

/home/Alexander_Pruss/pypolycsg/third_party/carve/include/carve/geom_impl.hpp:546:40: error: expected ‘,’ or ‘...’ before numeric constant
     plane<ndim>::plane(const vector_t &_N, vector_t _p) : N(_N), d(-dot(_p, _N)) {
                                        ^
In file included from /home/Alexander_Pruss/pypolycsg/third_party/carve/include/carve/geom.hpp:367:0,
                 from /home/Alexander_Pruss/pypolycsg/third_party/carve/include/carve/aabb.hpp:21,
                 from /home/Alexander_Pruss/pypolycsg/third_party/carve/lib/aabb.cpp:22:

The problem seems to be that something that was included has defined _N, and several other variables that start with an underscore, as a numeric constant. Renaming the variables that start with an underscore in include/carve/geom*.hpp fixes the problems and results in a successful build.