configure: fix parsing issue on include_dir option

configure is stuck in an endless loop if '--include_dir' option is used
without a value:

$ ./configure --include_dir
./configure: line 506: shift: 2: shift count out of range
./configure: line 506: shift: 2: shift count out of range
[...]

Fix it splitting 'shift 2' into two consecutive shifts, and making the
second one conditional to the number of remaining arguments.

A check is also provided after the while loop to verify the include dir
exists; this avoid to produce an erroneous configuration.

Fixes: a9c3d70d90 ("configure: add options ability")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David Ahern <dsahern@kernel.org>

configure

@@ -485,7 +485,7 @@ usage()
 {
 	cat <<EOF
 Usage: $0 [OPTIONS]
-	--include_dir		Path to iproute2 include dir
+	--include_dir <dir>	Path to iproute2 include dir
 	--libbpf_dir		Path to libbpf DESTDIR
 	--libbpf_force		Enable/disable libbpf by force. Available options:
 				  on: require link against libbpf, quit config if no libbpf support
@@ -502,8 +502,9 @@ else
 	while true; do
 		case "$1" in
 			--include_dir)
-				INCLUDE=$2
-				shift 2 ;;
+				shift
+				INCLUDE="$1"
+				[ "$#" -gt 0 ] && shift ;;
 			--libbpf_dir)
 				LIBBPF_DIR="$2"
 				shift 2 ;;
@@ -523,6 +524,8 @@ else
 	done
 fi
 
+[ -d "$INCLUDE" ] || usage 1
+
 echo "# Generated config based on" $INCLUDE >$CONFIG
 quiet_config >> $CONFIG