Replace forward slashes by backslashes in BMI path for MSVC. (fmtlib#4344)

* Fix slashes in BMI path for MSVC builds
* Fix BMI path for MSVC builds when building with Ninja generator

Author: tkhyn

CMakeLists.txt

@@ -27,7 +27,13 @@ endfunction()
 # DEPRECATED! Should be merged into add_module_library.
 function(enable_module target)
   if (MSVC)
-    set(BMI ${CMAKE_CURRENT_BINARY_DIR}/${target}.ifc)
+    if(CMAKE_GENERATOR STREQUAL "Ninja")
+      # Ninja dyndep expects the .ifc output to be located in a specific relative path
+      file(RELATIVE_PATH BMI_DIR "${CMAKE_BINARY_DIR}" "${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/${target}.dir")
+    else()
+      set(BMI_DIR "${CMAKE_CURRENT_BINARY_DIR}")
+    endif()
+    file(TO_NATIVE_PATH "${BMI_DIR}/${target}.ifc" BMI)
     target_compile_options(${target}
       PRIVATE /interface /ifcOutput ${BMI}
       INTERFACE /reference fmt=${BMI})