BUILD: makefile: add FAILFAST to select the -Wfatal-errors behavior

BUILD: makefile: add FAILFAST to select the -Wfatal-errors behavior

-Wfatal-errors is set by default and is not supported on older compilers.
Since it's part of all the automatically detected flags, it's painful to
remove when needed. Also it's a matter of taste, some developers might
prefer to get a long list of all errors at once, others prefer that the
build stops immediately after the root cause.

The default is now back to no -Wfatal-errors, and when FAILFAST is set to
any non-empty non-zero value, -Wfatal-errors is added:

 $ make TARGET=linux-glibc USE_OPENSSL=0 USE_QUIC=1 FAILFAST=0 2>&1 | wc
    132     536    6111

 $ make TARGET=linux-glibc USE_OPENSSL=0 USE_QUIC=1 FAILFAST=1 2>&1 | wc
      8      39     362