Use -Werror when testing for -Wno-format-truncation
Clang does not know the -Wno-format-truncation option of (current) GCC,
but accepts unknown -W... options (exit core 0) but issues a warning
message on every invocation. So for example on macOS, where Clang is
used as "gcc", a new warning message was shown for every file to
compile, since we enabled -Wno-format-truncation in commit
1d527eaf:
warning: unknown warning option '-Wno-format-truncation' [-Wunknown-warning-option]
Clang no longer acceps unknown -W... options by enabling -Werror, which
this patch adds to the CFLAGS while testing for -Wno-format-truncation,
which fixes this issue.
This fixes commit
1d527eaf.