Log the value of LIB_DIR for doc builds (debug)

CHANGELOG.md

@@ -1,3 +1,9 @@
+Version 99:
+
+* Log the value of LIB_DIR for doc builds (debug)
+
+--------------------------------------------------------------------------------
+
 Version 98:
 
 * basic_fields::key_compare is noexcept

doc/Jamfile

@@ -85,6 +85,7 @@ if [ os.name ] = NT
     {
         SET LIB_DIR=$(2:B=:S=..)
         SET XML_OUTPUT=$(1:D)
+        echo "LIB_DIR=$(LIB_DIR)" > $(LIB_DIR)/doc/html/lib_dir.txt
         "$(.doxygen)" $(2)
     }
 }