configure: add options ability

There are more and more global environment variables that land everywhere
in configure, which is making user hard to know which one does what.
Using command-line options would make it easier for users to learn or
remember the config options.

This patch converts the INCLUDE variable to command option first. Check
if the first variable has '-' to compile with the old INCLUDE path
setting method.

Signed-off-by: Hangbin Liu <haliu@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>

configure

@@ -7,7 +7,7 @@
 #                           off: disable libbpf probing
 #   LIBBPF_DIR              Path to libbpf DESTDIR to use
 
-INCLUDE=${1:-"$PWD/include"}
+INCLUDE="$PWD/include"
 
 # Output file which is input to Makefile
 CONFIG=config.mk
@@ -486,6 +486,35 @@ endif
 EOF
 }
 
+usage()
+{
+	cat <<EOF
+Usage: $0 [OPTIONS]
+	--include_dir		Path to iproute2 include dir
+	-h | --help		Show this usage info
+EOF
+	exit $1
+}
+
+# Compat with the old INCLUDE path setting method.
+if [ $# -eq 1 ] && [ "$(echo $1 | cut -c 1)" != '-' ]; then
+	INCLUDE="$1"
+else
+	while true; do
+		case "$1" in
+			--include_dir)
+				INCLUDE=$2
+				shift 2 ;;
+			-h | --help)
+				usage 0 ;;
+			"")
+				break ;;
+			*)
+				usage 1 ;;
+		esac
+	done
+fi
+
 echo "# Generated config based on" $INCLUDE >$CONFIG
 quiet_config >> $CONFIG