Update comments in src/Makefile.defs

src/Makefile.defs
- Updated ifeq blocks comment
- Removed "disable this till further notice" comment

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

src/Makefile.defs

@@ -112,10 +112,9 @@ ifeq ($(findstring rc.fas.harvard.edu,$(HOSTNAME)),rc.fas.harvard.edu)
   endif
 endif
 
-# Note: Additional systems should be added as needed.
+# Note: ifeq blocks for additional systems should be added as needed.
 
-# Disable this till further notice (bmy, 11/1/21)
-## Exit if we can't find libfl.a (Flex library file)
+# Exit if we can't find libfl.a (Flex library file)
 ifeq ($(wildcard $(FLEX_LIB_DIR)/libfl.*),)
  $(error "Could not find the Flex library at $(FLEX_LIB_DIR)!")
 endif