UTF8 on Windows 10 using guile 2.27

Building Gnu Denemo for Windows has resulted in attempts to read files
with four byte chars failing
(string-length "=F0=9D=85=AC")
results in 4 not 1
Are there options that should be set somewhere in the build or
initialization that will enable naive Windows 10 users to run the
program successfully?

Richard Shann