configure: fix parsing issue on libbpf_dir option

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

$ ./configure --libbpf_dir
./configure: line 515: shift: 2: shift count out of range
./configure: line 515: 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 libbpf dir
exists; also, as LIBBPF_DIR does not have a default value, configure bails
out if the user does not specify a value after --libbpf_dir, thus avoiding
to produce an erroneous configuration.

Fixes: 7ae2585b86 ("configure: convert LIBBPF environment variables to command-line options")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David Ahern <dsahern@kernel.org>

configure

@@ -486,7 +486,7 @@ usage()
 	cat <<EOF
 Usage: $0 [OPTIONS]
 	--include_dir <dir>	Path to iproute2 include dir
-	--libbpf_dir		Path to libbpf DESTDIR
+	--libbpf_dir <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
 				  off: disable libbpf probing
@@ -506,8 +506,9 @@ else
 				INCLUDE="$1"
 				[ "$#" -gt 0 ] && shift ;;
 			--libbpf_dir)
-				LIBBPF_DIR="$2"
-				shift 2 ;;
+				shift
+				LIBBPF_DIR="$1"
+				[ "$#" -gt 0 ] && shift ;;
 			--libbpf_force)
 				if [ "$2" != 'on' ] && [ "$2" != 'off' ]; then
 					usage 1
@@ -525,6 +526,9 @@ else
 fi
 
 [ -d "$INCLUDE" ] || usage 1
+if [ "${LIBBPF_DIR-unused}" != "unused" ]; then
+	[ -d "$LIBBPF_DIR" ] || usage 1
+fi
 
 echo "# Generated config based on" $INCLUDE >$CONFIG
 quiet_config >> $CONFIG