Add the iwyu target to get information on unused headers

With this change, building the iwyu target will provide information
on unused headers, which can be removed to reduce compilation times.

CMakeLists.txt

@@ -27,6 +27,9 @@
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #
 # Parts of this file are originally copyright (c) 2012-2013 The Cryptonote developers
+
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
 list(INSERT CMAKE_MODULE_PATH 0
   "${CMAKE_SOURCE_DIR}/cmake")
 include(CheckCCompilerFlag)
@@ -1048,3 +1051,13 @@ option(INSTALL_VENDORED_LIBUNBOUND "Install libunbound binary built from source
 
 
 CHECK_C_COMPILER_FLAG(-std=c11 HAVE_C11)
+
+find_package(PythonInterp)
+find_program(iwyu_tool_path NAMES iwyu_tool.py iwyu_tool)
+if (iwyu_tool_path AND PYTHONINTERP_FOUND)
+  add_custom_target(iwyu
+    COMMAND "${PYTHON_EXECUTABLE}" "${iwyu_tool_path}" -p "${CMAKE_BINARY_DIR}" -- --no_fwd_decls
+    COMMENT "Running include-what-you-use tool"
+    VERBATIM
+  )
+endif()