#The module is pcntl.so
apt-get install dpkg-dev make php7.0-dev
cd /tmp/phpsource
apt-get source php7.0 
cd /tmp/phpsource/php7.0-7.0.33/ext/pcntl/modules/
apt-get install phpize
phpize
./configure
apt-get install make
make
make test
cp /tmp/phpsource/php7.0-7.0.33/ext/pcntl/modules/pcntl.so /usr/lib/apache2/modules/
echo 'extension=pcntl.so' > /etc/php/7.0/mods-available/pcntl.ini
systemctl restart apache2