What configure is doing is running through a bunch of checks to make sure packages and programs required by wu-ftp are present. When you install from source you will eventually need to compile that source code. In this case you need to have a c compiler installed on your system. gcc is typically installed with linux distributions, but for whatever reason it is not on your system.
You can do one of two things. Either install gcc and all the components it requires or install wu-ftp from a binary package. This would be done using a package manager like rpm, apt, or yast. This all depends on your distribution. It is a good idea to enter what distribution you are using so people on the forum can better answer your questions. Go to the User CP and add your distribution by editing your profile. I would actually suggest using vsftpd, but that's just one persons opinion. It is quick, secure, and doesn't chew up a lot of real estate.