To echo Ken's words there's nothing wrong with building from source but that will add to the work required to keep things up to date. Similarly if there are problems getting extensions installed then these are PHP questions not Moodle questions.
So I'd also recommend using one of the third party package repos to install PHP 7.x. It makes both keeping up-to-date and installing extensions easier.
Regardless of whether you're building from source or using packages you can list the installed extensions with php -m or also confirm if intl is installed with php -i | grep intl as Ken says. Do both of these list the extension?
Also Ken says "If you did install missing php extensions, apache service needs to be restarted for system to be aware of them": are you using Apache or Nginx? It's systemctl restart httpd to restart Apache. Do you know if you're using mod_php or PHP-FPM? It's systemctl restart php-fpm to restart PHP-FPM.
This may all seem complicated but an environment built from source is unique to your environment so we have to ask lots of questions. If it's built from packages both the environment and the steps to make changes are more generic.