Ainda estou tentando obter uma solução usando um repositório externo, como o @Jukka sugere, mas até que o fiz, consegui compilá-lo com o seguinte:
sudo -i
yum remove libmemcached.x86_64
yum install bzr.x86_64
yum install gcc-c++
mkdir -p /tmp/memcached
cd /tmp/memcached
wget https://launchpad.net/libmemcached/1.0/1.0.18/+download/libmemcached-1.0.18.tar.gz
tar -zxvf libmemcached-1.0.18.tar.gz
cd /tmp/memcached/
bzr branch lp:libmemcached
cd libmemcached
./bootstrap.sh autoreconf
NOTE YOU GET THE FOLLOWING ERROR BUT CONTINUE ANYWAY: "./bootstrap.sh:315: An attempt was made to set an invalid VENDOR="
cd ../libmemcached-1.0.18
./configure
make
make test
make install