Install mcrypt on php (linux) app service fails

I have a PHP|7.4 Linux App Service that needs good-old mcrypt from pecl. 

I have pear and pecl provided by the existing installation.

When I do 

pecl install mcrypt

I get a long run of 'configure' that ends in 

checking for mcrypt support... yes, shared
configure: error: mcrypt.h not found. Please reinstall libmcrypt.

 Other times, I've gotten 'config.h' not found. 

Does anybody know how to install mcrypt (1.0.4)  I know that it's deprecated, and we shouldn't use it, but we have legacy needs. 

Thanks