This avoids compilation erroring out on C4819 (Unicode handling issue in the
Visual Studio compiler), notably when running on Chinese, Japanese and Korean
(CJK) locales.

This also applies -utf-8 into the cflags passed into the various g-ir-scanner
command lines that are used to generate the *.gir files, where -utf-8 is
available, so that we don't get flooded with C4819 warnings during the
build, and therefore avoid potential mishaps, as C4819 is a real warning that
warngs us the code may be incorrectly built.