Use -Werror when testing for -Wno-format-truncation

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.