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+

configure

@@ -6327,6 +6327,9 @@ EOF
     # Issue has been fixed in MSVC v19.00.24218.
     check_cpp_condition windows.h "_MSC_FULL_VER >= 190024218" ||
         check_cflags -d2SSAOptimizer-
+    # enable utf-8 source processing on VS2015 U2 and newer
+    check_cpp_condition windows.h "_MSC_FULL_VER >= 190023918" &&
+        add_cflags -utf-8
 fi
 
 for pfx in "" host_; do