Unable to install any Ruby with RVM on MacOS 10.14.2 Mojave: Error running '__rvm_make -j4'

I ran into this issue as well. In my case it was due to MacPorts being outdated after I upgraded to (MacOS) Mojave.

I followed the steps listed on MacPorts' migration guide here: https://trac.macports.org/wiki/Migration

Closed and reopened Terminal (just b/c)

ran rvm install ruby and got Install of ruby-2.6.0 - #complete, hooray!

Do you have multiple versions of boost installed? I took the following steps to fix this (although I was using rbenv).

First I removed my xcode lib: rm -rf /Library/Developer/CommandLineTools

Then I reinstalled it: xcode-select --install

Then I ran brew ls and noticed I had two versions of boost. boost and [email protected]. I removed [email protected] and boost and reinstalled boost and made sure it was linked via brew link boost.

Seemed to work after that.