Added an option for supporting deprecated C++ constructs

What was the reason for these path changes? Didn't they work?

I see two issues with the above code (aside the fact that they don't really belong to this commit): you have the path to the linux headers in two places, not just one, and naming the directory "linux" is not good, as there are no "linux" headers, there are headers for some particular userspaces using linux kernel. One of those is "x86_64-linux-gnu", but there are others.

Generally speaking, I think the ParseGen should look for the headers in some particular directory (say, build/headers/), and append the target triplet to the path. If there's no such directory, it could check if the host is the same as the target, and if so, look for the headers in some standard place for that system.