Visual Studio builds: Use -utf-8 where available

Hi,

From the commit message:

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.

With blessings, thank you!