Update build script to automatically detect number of CPUs

[kekkokk]

Avoiding calling cmake at every ./installErizo.sh allow us to build only the changes that are made between every make and speed up our developing process.

If we want to clear cache with can always run installErizo with -d option

[jcague]

what's the issue with the previous way?

[kekkokk]

in this way cmake is called only the first time that the folder is created after a cleaonOject is called.
I noticed that if cmake is run every time generateProject is called, it build the enteire project every time instead of only the changes made in cpp.

[jcague]

Just run buildProject.sh if you implement changes inside existing cpp files, or run generateProject.sh if you add/remove files or dependencies. With this change it won't regenerate cmake structure if the directory already exists.

[kekkokk]

ok it makes sense.

[kekkokk]

I left all the things needed to do a "power" compile

but @jcague why in the "old" v2, installErizo.sh -e, even redoing the cmake, didn't recompile everything but only changes?

[jcague]

I don't remember exactly, we probably changed CMakeLists in some way by adding more deps, and it now recreates everything from scratch.

[jcague]

LGTM