add a simple Makefile to build and clean up

add missing macros, static inline functions, etc.
add README to illustrate the purpose of this repo.

Signed-off-by: Yonghong Song <yhs@fb.com>

src/Makefile

@@ -0,0 +1,23 @@
+# SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+TOPDIR = ..
+INCLUDES = -I. -I$(TOPDIR)/include -I$(TOPDIR)/include/uapi
+CFLAGS = $(INCLUDES) -O2 -Werror -Wall
+
+FEATURE_REALLOCARRAY := $(shell $(TOPDIR)/scripts/check-reallocarray.sh)
+ifneq ($(FEATURE_REALLOCARRAY),)
+  CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
+endif
+
+OBJFILES = bpf.o btf.o libbpf.o libbpf_errno.o netlink.o nlattr.o str_error.o
+
+all: libbpf.a
+
+libbpf.a: $(OBJFILES)
+	/bin/rm -f $@; /bin/ar rcs $@ $^
+
+%.o: %.c
+	gcc $(CFLAGS) -c $< -o $@
+
+clean:
+	/bin/rm -f libbpf.a $(OBJFILES)