RHEL5 linux RPMs only have libssh2 v0.17 available. This version appears to be too old to build zabbix with libssh2 support.
Zabbix should check the libssh2 version in the configure script to provide a more useful error.

checks_ssh.c: In function 'waitsocket':
checks_ssh.c:64: error: 'LIBSSH2_SESSION_BLOCK_INBOUND' undeclared (first use in this function)
checks_ssh.c:64: error: (Each undeclared identifier is reported only once
checks_ssh.c:64: error: for each function it appears in.)
checks_ssh.c:67: error: 'LIBSSH2_SESSION_BLOCK_OUTBOUND' undeclared (first use in this function)

Also, the configure option --with-ssh2=/path/to/new/ssh2 appears to ignore the PATH portion of the option. A manual fix
to the configure script is shown in the forum: http://www.zabbix.com/forum/showthread.php?t=14536