The file you downloaded contains a compiled version of Ghostscript, not source code, so the instructions for how to build from source code do not apply. This is also explained to some degree by the supplied README.txt
file.
Like Fredi said, you should normally use your distribution's packaged version. This is especially true if you aren't particularly familiar with Linux. The proper way to do that should be to run sudo yum install ghostscript
or something very similar.
If you really do want to build from source, go to the Ghostscript download page and download the package labelled "Ghostscript 9.18 Source" instead of the package labelled "Ghostscript 9.18 for Linux x86 (64 bit)". You should then be able to follow the instructions for building from source code.