On Mac OS, `realpath` is deficient in determining the actual filename
on-disk as it will simply provide the string you gave it if that file
exists, instead of returning the filename as it exists. Instead we
must read the directory entries for the parent directory to get the
canonical filename.
Since the Linux platform has a case sensitive file system, the header name should be lower case for cross compiling purposes. (On Linux, the mingw header is called ```windows.h```).
On case insensitive filesystems, we may have files in the working
directory that case fold to a name we want to write. Remove those
files (by default) so that we will not end up with a filename that
has the unexpected case.
