Add options from Weimer2018 especially errors for format-security and implicit-function-declaration

[david-a-wheeler]

Florian Weimer's 2018 article "Recommended compiler and linker flags for GCC" has a number of interesting ideas. We should review it for options.

At the very least, I like adding these options:

-Werror=format-security \
-Werror=implicit-function-declaration

As already noted in the guide, adding a general -Werror for all warnings has a lot of negatives, as it makes compilation very dependent on a specific version of a specific compiler. However, adding specific warnings for specific cases does not have this downside. We should probably do more of it.

In this case, these turn "should never do that" cases into outright errors. Raising an error for implicit function declaration is also part of #339. I worry that autoconf might have some implicit function declarations (at least for exit()) - we may need to determine how to deal with that and document it.

Possible other options, though I'm not as confident:

-fasynchronous-unwind-tables \
-fexceptions

Some discussion about implicit function breakage is here:
https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213

[david-a-wheeler]

Looks like the latest version of autoconf will address some of these. It's planned to be released next week (December 2023). A discussion about this should soon show here: https://lists.gnu.org/archive/html/autoconf/2023-12/threads.html