cmake: integrate iwyu, if/when present

CMakeLists.txt

@@ -227,6 +227,22 @@ add_executable(fortuna ${FORTUNA_SOURCES} ${FORTUNA_HEADERS})
 target_include_directories(fortuna PRIVATE .)
 target_compile_features(fortuna PUBLIC cxx_std_20)
 
+if(CMAKE_BUILD_TYPE MATCHES "Debug")
+  message(STATUS "Looking for iwyu...")
+  find_program(iwyu NAMES include-what-you-use iwyu)
+  if(iwyu)
+      message(STATUS "iwyu found at: ${iwyu}")
+    set(iwyu_and_options
+      ${iwyu}
+      -Xiwyu
+      -p .
+      --verbose=3)
+    set_property(TARGET fortuna PROPERTY CXX_INCLUDE_WHAT_YOU_USE ${iwyu_and_options})
+  else()
+    message(STATUS "iwyu, not found")
+  endif()
+endif()
+
 # ref: https://cmake.org/pipermail/cmake/2016-May/063400.html
 target_link_libraries(fortuna
     PRIVATE cryptopp