cparser.py:_preprocess() wrongly strips // in #line paths

We've hit this as a build failure for python-zstandard with compiler finding fix applied. See also: https://bugs.gentoo.org/723476.

If gcc preprocessor is run with -g, it outputs the following additional #line statement (marked with ->):

   # 1 "/tmp/tmpxbwuq2vh.h"
-> # 1 "/tmp/python-zstandard//"
   # 1 "<built-in>"
   # 1 "<command-line>"
   # 1 "/usr/include/stdc-predef.h" 1 3 4

It seems that the _preprocess() function comment stripping regex wrongly matches that // as a comment. As a result, csource after substitution becomes:

   # 1 "/tmp/tmpxbwuq2vh.h"
-> # 1 "/tmp/python-zstandard
   # 1 "<built-in>"
   # 1 "<command-line>"
   # 1 "/usr/include/stdc-predef.h" 1 3 4

This in turn causes cpyparser to bail on invalid #line syntax.