The cross and native files should be written as utf-8 because that is what Meson expects, and we should read meson_options.txt as utf-8, because that is what it will always be.
Fixes the following error on macOS while reading meson_options.txt:
Traceback (most recent call last):
File "cerbero/build/oven.py", line 451, in _cook_recipe_step
await ret
File "cerbero/build/recipe.py", line 82, in async_wrapped
await stepfunc()
File "cerbero/build/build.py", line 63, in async_call
res = await func(*args)
File "cerbero/build/build.py", line 1011, in configure
self._set_option({'introspection', 'gir'}, 'gi')
File "cerbero/build/build.py", line 771, in _set_option
options = f.read()
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/encodings/ascii.py", line 26, in decode
return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 12: ordinal not in range(128)