Speeding up cgo builds

The go tool is used to build all of the CockroachDB source, including C and C++ dependencies via the cockroachdb/c-{rocksdb,snappy,protobuf} packages. Unfortunately, go build does not parallelize building of source files within a package. For .go files this isn't a problem because the Go compiler is fast. For C and C++ files the issue is more noticeable. Compiling the c-rocksdb package takes ~2min on my machine. Parallelizing the compilation of C and C++ files by go build reduces the build time to 30sec. If you frequently edit any of the cockroachdb/c-* packages and you're comfortable running a hacked version of the go tool, this patch is for you:

• Go1.6
• Go1.6.2
• Go1.7beta1