The docs for CCompiler.compile say:
> If output_dir is given, object files will be put under it, while retaining their
> original path component. That is, foo/bar.c normally compiles to foo/bar.o (for a
> Unix implementation); if output_dir is build, then it would compile to build/foo/bar.o.
What they forget to say is that path components are also retained if output_dir is not
specified, it just means it will do so in the current directory. So if you compile
a temporary C file /tmp/foo.c, it will produce a ./tmp/foo.o file relative to the
current directory.
This commit fixes that issue by passing an explicit output_dir itself located in
a temporary directory.