Clarify requirement of a g++ compiler to run setup_deps

[Portals]

Hello!

While trying to set up the project I ran into an issue where com.ibm.wala.cast didn't compile properly. After reading through the mvn output, I found out that I was missing a g++ compiler. I think this needs to be clarified in README.md.

With Ubuntu, I could retrieve a g++ compile with sudo apt install build-essential.

[parttimenerd]

Thanks for your comment. I did not know that.
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