[3870] Added a comment to explain the guard

src/bin/dhcp4/dhcp4_srv.h

@@ -33,6 +33,10 @@
 #include <iostream>
 #include <queue>
 
+// Undefine the macro OPTIONAL which is defined in some operating
+// systems but conflicts with a member of the RequirementLevel enum in
+// the server class.
+
 #ifdef OPTIONAL
 #undef OPTIONAL
 #endif

src/bin/dhcp6/dhcp6_srv.h

@@ -32,6 +32,10 @@
 #include <iostream>
 #include <queue>
 
+// Undefine the macro OPTIONAL which is defined in some operating
+// systems but conflicts with a member of the RequirementLevel enum in
+// the server class.
+
 #ifdef OPTIONAL
 #undef OPTIONAL
 #endif