Fix building RocksDB in paths containing spaces

RocksDB won't build if you happen to have checked it out in a path containing spaces.

```{sh}
$ mkdir ~/with\ spaces
$ cd ~/with\ spaces
$ git clone https://github.com/facebook/rocksdb.git
$ make -C rocksdb
```
```
/bin/sh: 1: export: spaces/rocksdb: bad variable name
make: Entering directory `/home/mlin/with spaces/rocksdb'
Makefile:24: build_config.mk: No such file or directory
make: *** No rule to make target `build_config.mk'.  Stop.
make: Leaving directory `/home/mlin/with spaces/rocksdb'
```

This commit fixes it by quoting some path names in `Makefile` and `build_detect_platform`.