giscanner/sourcescanner.py: Use Tempfiles For Parsing

As we attempt to move giscanner to use distutils for preprocessing and
building the introspection dumper program, we need to update _parse(),
where the preprocessor is called, to not use stdin and stdout for the
preprocessor input and output, as distutils do not accept such usage.

The added bonus for this change is that MSVC builds can be done without
using MinGW/GCC as a helper, as the '-' flag for preprocessing from stdin
was the hindrance to that.

https://bugzilla.gnome.org/show_bug.cgi?id=728313