You can build Harvey with clang if you prefer, or possibly even pcc. Getting Harvey compiling with GCC/clang meant translating Plan 9's C dialect into ISO C. The translation process means there wasn't a lot of opportunity to introduce GCCisms, so any standards-compliant C compiler should be able to build Harvey. I say should because you may run into one or two small nits that have to be fixed before it'll work, and of course you have to find an up-to-date C compiler that's not GCC or Clang (I guess it would be good to try icc?)