I've realized that I get the OpenBLAS error regardless of whether or not I set OPENBLAS_DIR=BUILD. So I think there's something wrong with my optionlist.
The optionlist worked previously, but it's been a while since I tried compiling the Einstein Toolkit on my laptop. In between the successful compiles and now, I installed gcc-6 via MacPorts (which is currently my default version). Perhaps this could be causing trouble.
My laptop is a bit of a mess: I'm using both MacPorts and Homebrew, have multiple versions of things, and am generally unsure about what is linked to what. I'm actually tempted to do a factory reset, choose a single package manager (and stick to it), and do fresh checkouts of everything I need.