Patch PA4 for missing yylex

libfl is missing yylex symbol. Since libfl is not necessary for the semantic
analyzer, just remove `-lfl` from the Makefile.

Dockerfile

@@ -85,7 +85,8 @@ RUN sed -i 's/-zx/-ozx/g' \
 #   c) fix mangling errors of yylex when compiled with a c++ compiler
 #   d) be as non-invasive as possible to the existing assignment code
 RUN patch -u /usr/class/assignments/PA2/cool.flex -i /tmp/build/cool.flex.patch \
-    && patch -u /usr/class/assignments/PA3/cool.y -i /tmp/build/cool.y.patch
+    && patch -u /usr/class/assignments/PA3/cool.y -i /tmp/build/cool.y.patch \
+    && patch -u /usr/class/assignments/PA4/Makefile -i /tmp/build/Makefile.patch
 
 # Setup working directory
 RUN mkdir -p /class

Makefile.patch

@@ -0,0 +1,11 @@
+--- Makefile	2021-01-03 12:11:07.454989428 +0000
++++ Makefile.work	2021-01-03 12:15:19.965132466 +0000
+@@ -2,7 +2,7 @@
+ ASSN = 4
+ CLASS= cs143
+ CLASSDIR= ../..
+-LIB= -L/usr/pubsw/lib -lfl 
++LIB= -L/usr/pubsw/lib
+ AR= gar
+ ARCHIVE_NEW= -cr
+ RANLIB= gar -qs