Don't make bin dir if it exists.

mkdir \`dirname ../bin/streem\` command raises an error if bin directory exists.

```
$ make
mkdir `dirname ../bin/streem`
cc -g parse.o -o ../bin/streem
$ make clean
rm -f y.output y.tab.c
rm -f lex.yy.c
rm -f *.o ../bin/streem
$ make
bison -y parse.y
flex lex.l
cc -g -c y.tab.c -o parse.o
mkdir `dirname ../bin/streem`
mkdir: ../bin: File exists
make: *** [../bin/streem] Error 1
```