build: use DEFINED to test the existence of the MSVC variable

build-support/custom-modules/ns3-vcpkg-hunter.cmake

@@ -73,7 +73,7 @@ function(setup_vcpkg)
     )
   endif()
 
-  if(${MSVC})
+  if(DEFINED MSVC)
     message(FATAL_ERROR "vcpkg: Visual Studio is unsupported")
   else()
     # Check if required packages are installed (unzip curl tar)

build-support/macros-and-definitions.cmake

@@ -205,7 +205,7 @@ macro(process_options)
 
   # Set warning level and warning as errors
   if(${NS3_WARNINGS})
-    if(MSVC)
+    if(DEFINED MSVC)
       add_compile_options(/W3) # /W4 = -Wall + -Wextra
       if(${NS3_WARNINGS_AS_ERRORS})
         add_compile_options(/WX)