CMake: option FOUR_C_ENABLE_IWYU

Runs iwyu alongside the build and reports fixes to header includes and
forward declarations. Currently, more a developer option, could be
enforced when the code is cleaner in that regard.

Author: sebproell

cmake/functions/four_c_auto_define_module.cmake

@@ -38,6 +38,12 @@ function(four_c_auto_define_module)
     # them on other users of the library.
     target_link_libraries(${_target}_objs PRIVATE four_c_private_compile_interface)
 
+    if(FOUR_C_ENABLE_IWYU)
+      set_target_properties(
+        ${_target}_objs PROPERTIES CXX_INCLUDE_WHAT_YOU_USE ${FOUR_C_IWYU_EXECUTABLE}
+        )
+    endif()
+
     if(FOUR_C_ENABLE_DEVELOPER_MODE AND _parsed_NO_CYCLES)
       # Define an additional library built from the sources. In developer mode, we link unit tests against this library which can
       # be faster to build than lib4C.

cmake/setup_global_options.cmake

@@ -152,6 +152,17 @@ else()
   enable_compiler_flag_if_supported("-fno-trapping-math")
 endif()
 
+four_c_process_global_option(FOUR_C_ENABLE_IWYU "Enable include-what-you-use" OFF)
+if(FOUR_C_ENABLE_IWYU)
+  find_program(FOUR_C_IWYU_EXECUTABLE NAMES include-what-you-use iwyu)
+  if(NOT FOUR_C_IWYU_EXECUTABLE)
+    message(
+      FATAL_ERROR "Option FOUR_C_ENABLE_IWYU is ON but include-what-you-use/iwyu is not found."
+                  "You can specify the path in the CMake variable FOUR_C_IWYU_EXECUTABLE."
+      )
+  endif()
+endif()
+
 ##
 # Optimization flags
 # These flags are reasonable defaults. Users may amend them by setting FOUR_C_CXX_FLAGS and/or FOUR_C_CXX_FLAGS_<CONFIG>.