Introduce libi3, an *internal* library to eliminate code duplication

libi3/Makefile

@@ -0,0 +1,26 @@
+# Default value so one can compile i3-msg standalone
+TOPDIR=..
+
+include $(TOPDIR)/common.mk
+
+CFLAGS += -I$(TOPDIR)/include
+
+# Depend on the object files of all source-files in src/*.c and on all header files
+FILES=$(patsubst %.c,%.o,$(wildcard *.c))
+HEADERS=$(wildcard *.h)
+
+# Depend on the specific file (.c for each .o) and on all headers
+%.o: %.c ${HEADERS}
+	echo "[libi3] CC $<"
+	$(CC) $(CPPFLAGS) $(CFLAGS) -c -o $@ $<
+
+all: libi3.a
+
+libi3.a: ${FILES}
+	echo "[libi3] AR libi3.a"
+	ar rcs libi3.a ${FILES}
+
+clean:
+	rm -f *.o libi3.a
+
+distclean: clean