Bug#34309: '_PC' macro redefinition
For reasons that are now a mystery, we had defined a CPP symbol to
help ancient compilers work better (in some way that's lost to history).
This interferes with at least one modern compiler.
Now, don't define the _PC symbol. Those other underscore-leading
symbols are suspect also, but at least the names aren't inscrutable.
Let's leave them for now.