The file
/tmp/libmcrypt-2.5.8.tar.gz
is almost certainly a source code tar-ball, not a RedHat RPM. For RedHat based distros, libmcrypt is part of epel, so you will have to do something like
sudo dnf update
sudo install epel-release
sudo install libmcyrpt-devel
That will install not only libmcrypt headers, but also libmcrypt, and any package dependencies.
To build mcrypt from source the procedure is something like:
tar xf /tmp/libmcypt-2.5.8.tar.gz
cd libmcrypt.2.5.8
./configure
make
sudo make install
That will probably install under /usr/local, so you may have to either add
LD_LIBRARY_PATH=/usr/local/lib64
to your .profile, or add an entry /usr/local/lib64 to /etc/ld.config.d/ and then
sudo ldconfig