Please be aware that current USE flags and other configuration defined in /etc/make.conf
may be have changed since the package was installed. A more reliable way of checking configuration and compile options is to check var/db/pkg
.
Example for apache-2.2.22:
$ cat /var/db/pkg/www-servers/apache-2.2.22-r1/CFLAGS
-mtune=native -O2 -pipe -g