Source: libv8 fails to build on Catalina 10.15 #282

Another one of those troublesome gems, here is my definite guide to installing therubyracer, along with an older v8 from brew.

env \
  CXX=clang++ \
  GYPFLAGS=-Dmac_deployment_target=10.9 \
gem install libv8 --version 3.16.14.19