Add shared library make target for C library

c/Makefile

@@ -59,12 +59,15 @@ all: $(MAINS)
 
 # Delete build output
 clean:
-	rm -f -- $(MAINS)
+	rm -f -- $(MAINS) libqrcodegen.so
 
 # Executable files
 %: %.c $(LIBSRC:=.c) $(LIBSRC:=.h)
 	$(CC) $(CFLAGS) -o $@ $< $(LIBSRC:=.c)
 
+libqrcodegen.so: $(LIBSRC:=.c) $(LIBSRC:=.h)
+	$(CC) $(CFLAGS) -fPIC -shared -o $@ $<
+
 # Special executable
 qrcodegen-test: qrcodegen-test.c $(LIBSRC:=.c) $(LIBSRC:=.h)
 	$(CC) $(CFLAGS) -DQRCODEGEN_TEST -o $@ $< $(LIBSRC:=.c)