build: CMake and ns3 fixes

Includes:
- print error message instead of forwarding posix signals in ns3
- supress printing of "Finished executing commands..." for ./ns3 run
- fix ns3 typos and formatting issues
- add verbose options and make doxygen/doxygen-no-build verbose
- re-enable printing of build messages in ./ns3 run
- refactor ns3 dry_run, quiet, jobs and verbose arguments
- check if examples subdirectories have a CMakeLists.txt

examples/CMakeLists.txt

@@ -3,6 +3,9 @@ if(${ENABLE_EXAMPLES})
 
   # Process subdirectories
   foreach(examplefolder ${examples_to_build})
+    if(NOT (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${examplefolder}/CMakeLists.txt))
+      continue()
+    endif()
     add_subdirectory(${examplefolder})
 
     set(ns3-example-folders