configure: instruct MSVC 2015 to properly process UTF-8 string literals

Without the /UTF-8 switch, the MSVC compiler treats all files as in the
system codepage, instead of in UTF-8, which causes UTF-8 string literals
to be interpreted wrong.

This switch was only introduced in VS2015 Update 2, and any earlier
versions do not have an equivalent solution.

Fixes fate-sub-scc on MSVC 2015+