Adding more APACHE_MODULES options in Gentoo's make.conf will replace the defaults.  Then you have to manually add all the defaults back in.     :-(

• Instead of enabling them with APACHE2_MODULES in make.conf, I added them to /etc/portage/package.use and prefixed each with "apache2_modules_"
  
  So, in my package.use, I have this:
  
  www-servers/apache ssl apache2_modules_proxy apache2_modules_proxy_http (etc)
  
  This way, the list of default modules is preserved.

• Alternatively, this works in make.conf:
  
  APACHE2_MODULES="${APACHE2_MODULES} extra_module1 extra_module2"

(From: http://bugs.gentoo.org/show_bug.cgi?id=208108#c4  &  http://bugs.gentoo.org/show_bug.cgi?id=208108#c5 )