build: Make sure CMAKE_CONFIGURATION_TYPES is filled

build-support/custom-modules/ns3-platform-support.cmake

@@ -81,6 +81,9 @@ endif()
 if(${MSVC} OR ${XCODE})
   # Prevent multi-config generators from placing output files into per
   # configuration directory
+  if(NOT (DEFINED CMAKE_CONFIGURATION_TYPES))
+    set(CMAKE_CONFIGURATION_TYPES DEBUG RELEASE RELWITHDEBINFO MINSIZEREL)
+  endif()
   foreach(OUTPUTCONFIG ${CMAKE_CONFIGURATION_TYPES})
     string(TOUPPER ${OUTPUTCONFIG} OUTPUTCONFIG)
     set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_${OUTPUTCONFIG}