cmake related changes.
* Add the toolchain files for building using gcc or llvm/clang.
  E.g.
  mkdir build
  cd build
  cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/llvm-toolchain.cmake -DCMAKE_INSTALL_PREFIX=install ..
  make install # install to build/install
* Some small changes to make both gcc and llvm build, although there are still some warnings in llvm.
Move setopt to Curl.h so applications can use it. And also turn off the EPOLL_CTL_DEL logging as it's quite annoying.