Rename C++ variables to avoid use of reserved identifiers

This is a bug fix for issue #2463.

The C++ triangulation code (_tri.h and _tri.cpp) uses variables with names _A, _C and _M.  However, in C++ identifiers that begin with an underscore followed by a capital letter are reserved.  This PR changes such variables to their lowercase equivalents.

I have confirmed that following the changes the triangulation tests pass OK and there is no change to any of the triangulation examples.