Replace forward slashes by backslashes in BMI path for MSVC.

Note that this is broken on Ninja

CMakeLists.txt

@@ -27,7 +27,8 @@ endfunction()
 # DEPRECATED! Should be merged into add_module_library.
 function(enable_module target)
   if (MSVC)
-    set(BMI ${CMAKE_CURRENT_BINARY_DIR}/${target}.ifc)
+    string(REPLACE "/" "\\" BIN_DIR ${CMAKE_CURRENT_BINARY_DIR})
+    set(BMI ${BIN_DIR}\\${target}.ifc)
     target_compile_options(${target}
       PRIVATE /interface /ifcOutput ${BMI}
       INTERFACE /reference fmt=${BMI})