Well... I didn't have to do that with PHP 7.3.
But, that's not to say that the way I was doing it with PHP 7.3 was correct. Perhaps it wasn't and it just so happened to work.
Ideally, I don't really want the custom cURL to be associated with anything system related. This version of cURL is compiled specifically intended to use with PHP only. That's what I liked about the way I could compile this with PHP 7.3 (and 7.2 and 7.1), I could "hide" the custom cURL and compile the PHP modules against it.
I can kind of get around this by manually modifying the CURL_SHARED_LIBADD and INCLUDES variables in the resulting Makefile before issuing the make command.
Just seems to me that PHP 7.4's libcurl is missing the option to specify a directory path with the --with-curl parameter.