Meson does only emit a warning if unknown options are passed. In our
case, this is always something we should take care of, because an
option we assumed we configured, had no effect.

See also https://github.com/mesonbuild/meson/pull/8658

Since only newer versions of IWYU_TOOL fail with an exit code, we have
to disable -e around the IWYU_TOOL invocation. Otherwise we would not
print IWYU_LOG.

This required that we backport iwyu_tool.py from
https://github.com/include-what-you-use/include-what-you-use/pull/891
into tools/, which supports --load.

Fixes #8.