The version of clang included in Apple LLVM 9.1.0 complains about
passing the bool parameter `is_error` to va_start, which causes make
to fail:
fdbrpc/TLSConnection.actor.cpp:370:16: error: passing an object that undergoes
default argument promotion to 'va_start' has undefined behavior
[-Werror,-Wvarargs]
va_start( ap, is_error );
^
This just switches is_error back to the type it gets promoted to (int).
Alvin Moore