Add -p option for mkdir to suppress error.

Add -p option for mkdir to suppress error.

Compiling nkf in Docker, error occur and ignored by make.
https://github.com/conao3/docker-nkf

```
Step 6/13 : RUN git clone https://github.com/nurse/nkf.git
 ---> Using cache
 ---> 1f18636e4d38
Step 7/13 : WORKDIR nkf
 ---> Using cache
 ---> e0210ecbf6aa
Step 8/13 : RUN make
 ---> Using cache
 ---> 62e20c0b3115
Step 9/13 : RUN make install
 ---> Running in 79714e0da6e0
mkdir: can't create directory '/usr/local/bin': File exists
make: [Makefile:49: install-main] Error 1 (ignored)
```

mkdir has option -p and it suppress error if directory
has existed.

```
-p, --parents
    no error if existing, make parent directories as needed
```