I had the same problem with memcache and memcached modules on MAMP 3.5.2 with PHP 7.0.8 and was able to install both modules from source code. For memcache, I used the websupport-sk git repo . Use it in your own risk. Here is steps that I followed for memcache installation
sudo su
cd /private/tmp
wget https://github.com/websupport-sk/pecl-memcache/archive/NON_BLOCKING_IO_php7.zip
unzip NON_BLOCKING_IO_php7.zip
cp -r pecl-memcache-NON_BLOCKING_IO_php7 /Application/MAMP/bin/php/php7.0.8/include/php/ext/memcache
cd /Application/MAMP/bin/php/php7.0.8/include/php/ext/memcache
phpize
./configure --enable-memcache --with-php-config=/Application/MAMP/bin/php/php7.0.8/bin/php-config
make
cp modules/memcache.so /Applications/MAMP/bin/php/php7.0.8/lib/php/extensions/no-debug-non-zts-20151012/memcache.so
Add following lines to php.ini
[memcache]
memcache_extension="/Applications/MAMP/bin/php/php7.0.8/lib/php/extensions/no-debug-non-zts-20151012/memcache.so"
extension=memcache.so
Hope this helps!