ruby with jemalloc won't configure - homebrew/OSX/montery/m1/arm(?)

Question

OSX/Monterey/M1/Arm/Homebrew: can't for the life of me get this to work:

./configure --with-jemalloc

It dies with

checking jemalloc/jemalloc.h presence... no
configure: error: jemalloc requested but not found

even though it's right there in: /opt/homebrew/Cellar/jemalloc/5.2.1_1/include/jemalloc/jemalloc.js.

brew --prefix jemalloc and jemalloc-config --includedir all looks correct.

I've tried various versions of:

export RUBY_CONFIGURE_OPTS="--with-jemalloc-include=$(brew --prefix jemalloc)/include

and/or

./configure --with-jemalloc-include=$(brew --prefix jemalloc)/include

but no dice. Not even sure those env vars/flags are respected by ruby or if they're some rvm/rbenv/ruby-build thing.

I'm down to doing all my testing directly with ruby's configure and not a rvm/build tool.