Adds -p option so that `mkdir` does not error if build dir already exists.
Before this change, if chia_bls build failed for some previous reason (e.g. b/c cmake was not installed), the chia_bls build would always fail in the future because the `build` directory already exists. Previous error message:
```
$ make -j4
Configuring chia_bls...
mkdir: cannot create directory ‘build’: File exists
make: *** [/Users/nmarley/projects/dash/depends/work/build/x86_64-apple-darwin18.0.0/chia_bls/b24c15cef6567a855e901b4774d1d22efb063ea9-e5023308c2a/./.stamp_configured] Error 1
```