For those looking to install OSSEC on the OLPC (One Laptop per Child), I was able to do it without major issues. Their system is based on the Fedora 7, so I just had to install make, gcc and the libc-headers.
A quick sequence of commands:
# yum -y install gcc make glibc-headers glibc-dev
# wget http://www.ossec.net/files/ossec-hids-1.5.1.tar.gz
# tar -zxvf ossec-hids-1.5.1.tar.gz
# cd ossec-hids-1.5.1