Only set the Makefile variables conditionally so that they may be
overridden by the FreeBSD port.

--- source/Makefile.orig
+++ source/Makefile
@@ -26,18 +26,19 @@
 #
 
 # GCC compiler and linker flags.
-CC	= gcc
-DEBUG	=
-CEXTRA	=
-CFLAGS	= -O2 -D__NO_STRING_INLINES -fPIC -ansi -D_STRICT_ANSI_ -Wall -W   \
+CC	?= gcc
+DEBUG	?=
+CEXTRA	?=
+CFLAGS	?= -O2 -D__NO_STRING_INLINES -fPIC -ansi -D_STRICT_ANSI_ -Wall -W  \
 	  -pedantic -Wshadow -Wpointer-arith -Wstrict-prototypes	   \
 	  -Wmissing-prototypes -Wmissing-declarations -Wredundant-decls	   \
 	  -Wwrite-strings -Wundef -Wbad-function-cast -Waggregate-return   \
 	  -Wnested-externs						   \
-	  $(DEBUG) $(CEXTRA)
-LDLIBS	=
-LDEXTRA	=
-LDFLAGS	= $(DEBUG) $(LDEXTRA)
+	  $(DEBUG)
+CFLAGS	+= $(CEXTRA)
+LDLIBS	?=
+LDEXTRA	?=
+LDFLAGS	?= $(DEBUG) $(LDEXTRA)
 
 # Generic compiler and linker flags.
 #CC	= cc
@@ -79,9 +80,9 @@
 SHELL	= /bin/sh
 
 # Miscellaneous tools, overridable by the command line.
-UNZIP	= unzip
-RM	= rm
-TEST	= test
+UNZIP	?= unzip
+RM	?= rm
+TEST	?= test
 
 # SCARE objects and headers.
 OBJECTS	= sctafpar.o sctaffil.o scprops.o scvars.o scexpr.o scprintf.o	\