build: reduce scope of NS3_ENABLE_EXAMPLES flag

Prevents recompilation of modules when tests are enabled

build-support/custom-modules/ns3-module-macros.cmake

@@ -97,6 +97,19 @@ function(build_lib)
            ${CMAKE_HEADER_OUTPUT_DIRECTORY}/version-defines.h
       )
     endif()
+
+    # Enable examples as tests suites
+    if(${ENABLE_EXAMPLES} AND ${ENABLE_TESTS})
+      if(NOT ${XCODE})
+        target_compile_definitions(
+          ${lib${BLIB_LIBNAME}}-obj PRIVATE NS3_ENABLE_EXAMPLES
+        )
+      else()
+        target_compile_definitions(
+          ${lib${BLIB_LIBNAME}} PRIVATE NS3_ENABLE_EXAMPLES
+        )
+      endif()
+    endif()
   endif()
   set_target_properties(
     ${lib${BLIB_LIBNAME}}

build-support/macros-and-definitions.cmake

@@ -1156,12 +1156,6 @@ macro(process_options)
     add_definitions(-DNS3_ASSERT_ENABLE)
   endif()
 
-  # Enable examples as tests suites
-  if(${ENABLE_EXAMPLES})
-    set(NS3_ENABLE_EXAMPLES "1")
-    add_definitions(-DNS3_ENABLE_EXAMPLES -DCMAKE_EXAMPLE_AS_TEST)
-  endif()
-
   set(ENABLE_TAP OFF)
   if(${NS3_TAP})
     set(ENABLE_TAP ON)