deps script for Centos 7

I wanted to do some testing with proxygen, which depends on fbthrift. I've checked in a script that gets fbthrift to build on Centos 7. Mostly I had to convert the apt packages to the equivalent yum packages. Additionally I had to tweak the LD_LIBRARY_PATH to get configure to finish successfully. configure was reporting that it couldn't find libevent when it actually couldn't find libfolly.so.18.