BUILD: makefile: add -Wfatal-errors to the default flags

BUILD: makefile: add -Wfatal-errors to the default flags

Some error reports are misleading on some recent versions of gcc because
it goes on to build for a very long time after it meets an error. Not
only this makes it hard to scroll back to the beginning of the error,
but it also hides the cause of the error when it's prominently printed
in a "#error" statement. This typically happens when building with QUIC
and without OPENSSL where there can be 4 pages of unknown types and such
errors after the "Must define USE_OPENSSL" suggestion.

The flag -Wfatal-errors serves exactly this purpose, to stop after the
first error, and it's supported on all the compilers we support, so let's
enable this now.