Fix build system to honour user flags.

--- a/Makefile
+++ b/Makefile
@@ -1,13 +1,11 @@
-
 # For better performance, replace ``-O'' with whatever
 # the best optimization flag is for your computer.
 # For Sun's compilers under Solaris, ``-fast'' works well.
 # For gcc, ``-O2'' works well.
-CC=cc
-CFLAGS=-O
-LDLIBS=-lm
+CC ?= gcc
+LDLIBS = -lm
  
 sim4:
-	$(CC) -o sim4 -I. $(CFLAGS) *.c $(LDLIBS)
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o sim4 -I. *.c $(LDLIBS)
 clean:
 	rm -f sim4 *.o