Fixes compilation error with clang
Fixes compilation error with clang

It seems that Clang doesn’t like #elif X abbreviated clauses. This
patch replace all of those by more verbose #elif defined(X) version

alobbs/libhpack,http2d/core,alobbs/libhpack,http2d/core,http2d/core,http2d/core,alobbs/libhpack,alobbs/libhpack