Fix crangler Makefile

This removes the use of CPROVER_DIR to find the common include, which in
turn enables setting this environment variable in repos that use the cbmc
repo as submodule.

Author: kroening

src/crangler/Makefile

@@ -15,7 +15,7 @@ OBJ += ../big-int/big-int$(LIBEXT) \
 INCLUDES= -I ..
 
 include ../config.inc
-include ../../$(CPROVER_DIR)/src/common
+include ../common
 
 CLEANFILES = crangler$(LIBEXT) \
   c_lex.yy.cpp \