cmake: scope /utf-8 to C and C++ so nvcc does not treat it as an input file

CMakeLists.txt

@@ -22,10 +22,13 @@ find_package(Threads REQUIRED)
 # Suppress MSVC fopen/sprintf deprecation warnings, force UTF-8 source and
 # execution charsets so non-ASCII string literals (CJK language names,
 # punctuation tables in BPE and prompt builder) survive the compile
-# without a BOM.
+# without a BOM. /utf-8 is restricted to C and C++ since nvcc treats a
+# bare /utf-8 as an input filename and aborts with "A single input file
+# is required". CUDA sources do not carry CJK literals so they do not
+# need this flag.
 if(MSVC)
     add_compile_definitions(_CRT_SECURE_NO_WARNINGS)
-    add_compile_options(/utf-8)
+    add_compile_options($<$<COMPILE_LANGUAGE:C,CXX>:/utf-8>)
 endif()
 
 # Put executables and backend .so in the same directory (build root).