the library can be compiled as a shared library

The shared library libdtree.so is preferred when compiling busio.
But if only libdtree.a exists, it is used instead.

Makefile

@@ -1,14 +1,18 @@
 CC = gcc
 AR = ar
-CFLAGS = -std=gnu99 -Wall -pedantic -Wextra -g
+CFLAGS = -std=gnu99 -Wall -pedantic -Wextra -g -fPIC
 
 Q ?= @
 
-all: libdtree.a
+all: libdtree.a libdtree.so
 libdtree.a: dtree_error.o dtree_procfs.o dtree.o bcd_arith.o
 	$(Q) $(AR) rcs $@ $^
 
-busio: busio.o libdtree.a
+libdtree.so: dtree_error.o dtree_procfs.o dtree.o bcd_arith.o
+	$(Q) $(CC) -shared -o $@ $^
+
+busio: busio.o
+	$(CC) $(LDFLAGS) $^ -L. -ldtree -o $@
 busio.o: busio.c
 
 clean: