Restore check for libfl library in Makefile.defs

Changed the error check to look for libfl.* in FLEX_LIB_DIR instead
of libfl.a.  On many systems libfl will be installed as a shared library
(i.e. libfl.so) instead of a static library (i.e. libfl.a).

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

src/Makefile.defs

@@ -1,4 +1,3 @@
-# -*- makefile-gmake -*-
 ##############################################################################
 #
 #  KPP - The Kinetic PreProcessor
@@ -117,8 +116,8 @@ endif
 
 # Disable this till further notice (bmy, 11/1/21)
 ## Exit if we can't find libfl.a (Flex library file)
-#ifeq ($(wildcard $(FLEX_LIB_DIR)/*.a),)
-# $(error "Could not find the Flex library at $(FLEX_LIB_DIR)!")
-#endif
+ifeq ($(wildcard $(FLEX_LIB_DIR)/libfl.*),)
+ $(error "Could not find the Flex library at $(FLEX_LIB_DIR)!")
+endif
 
 ##############################################################################