Enable multi-threading when building boost libraries

[baegjae]

When I ran eosio_build.sh, it takes too long time.
The cause was that only one thread is used when building the boost libraries.
I modified the script to use multiple threads.
Our time is precious :)

[heifner]

@jgiszczak What are your thoughts on this? Do you think it will cause more support requests than it is worth? Also it should be JOBS not CPU_CORE, I assume.

[baegjae]

@lazaridiscom, Thank you for your comment. When building the MongoDB driver, CPU_CORE was used as an argument within the build-scripts. Likewise, I thought CPU_CORE should be used when building the boost libraries.

[heifner]

See #4920. I think we should pass jobs # to the build script defaulted to 1. That would help with the build problems where people don't have enough ram. Let the user indicate how many cores they want to throw at it.