options: Fix long/short option mixture detection

[stoeckmann]

Take the key/value separator = into account when comparing long and short option names to avoid invalid matches.

Proof of Concept, run in top directory of checked out meson project:

./meson.py -Dunity=on --unity-size=4

Before this pull request, the output is:

Got argument unity as both -Dunity= and --unity. Pick one.

After this pull request, the output is:

ERROR: Must specify at least one directory name.
WARNING: Running the setup command as `meson [options]` instead of `meson setup [options]` is ambiguous and deprecated.

[jpakkane]

This needs a test.

[stoeckmann]

I have added a unit test.