If you are compiling v7.3.x under CentOS 8, you will need to first enable PowerTools, then install aspell-devel - otherwise aspell-devel will not be available to you:
sudo dnf config-manager --set-enabled PowerTools
sudo dnf install -y aspell-devel
Then in your configure line, just use:
--with-pspell
NOT
--with-pspell=/some/dir