Update CI to build both clang and gcc versions
As shown in #101 (closed), we might want to have CI build both a clang and gcc version on Linux for testing. This wouldn't have caught the error, but we can check to see if the different compilers produce different results. This certainly would have helped with #68 (closed) which would have shown that clang worked better. But ultimately #68 (closed) was caused by a bug in our code, so there's that.
This makes CI take longer, so not sure if we want to do that.