This has outlived its usefulness, doesn't gel well with
newer compilers & `-flto` related options, i.e thin vs full, or `=auto`,
and having `-flto` as the only option means that sometimes this just
needs to be worked around, i.e in oss-fuzz:
https://github.com/google/oss-fuzz/blob/master/projects/bitcoin-core/build.sh.
While it was convenient when `-flto` was newer, support for `-flto` is now
in all compilers we use, and there's also no-longer any real need
for us to treat `-flto` different to any other optimization option.
Remove it, to remove build complexity, and so there's no need
to port a similar option to CMake.
Note that the LTO option remains in depends, because we still a way to
build packages that have LTO specific patches/options.
If we decide to merge this, I'll follow up downstream in oss-fuzz first,
to make sure we don't break the build.
`_LIBCPP_ENABLE_ASSERTIONS` is deprecated, and will be removed. [See (from libc++ __config in main)](b57df9fe9a/libcxx/include/__config (L205-L209)):
> TODO(hardening): remove this in LLVM 19.
> This is for backward compatibility -- make enabling `_LIBCPP_ENABLE_ASSERTIONS` (which predates hardening modes)
> equivalent to setting the safe mode.
> ifdef _LIBCPP_ENABLE_ASSERTIONS
> warning "_LIBCPP_ENABLE_ASSERTIONS is deprecated, please use _LIBCPP_ENABLE_SAFE_MODE instead."
From LLVM 17, `_LIBCPP_ENABLE_DEBUG_MODE` can be used instead.
See https://libcxx.llvm.org/Hardening.html.
Related to #28476.
It was deprecated in LLVM 15, turned into a compile-time error in LLVM 16:
```bash
In file included from /usr/lib/llvm-16/bin/../include/c++/v1/cassert:19:
/usr/lib/llvm-16/bin/../include/c++/v1/__assert:22:5: error: "Defining _LIBCPP_DEBUG is not supported anymore.
Please use _LIBCPP_ENABLE_DEBUG_MODE instead."
^
1 error generated.
```
and has been removed entirely in LLVM 17 (main),
ff573a42cd.
Building libc++ in debug mode, will also automatically set
`_LIBCPP_ENABLE_DEBUG_MODE` (the new define), so adding it to depends
doesn't seem useful, and would just result in redefinition errors.
I'm wondering if as a followup, we could enable a DEBUG build of libc++
in our MSAN CI job?
Somewhat related to https://github.com/google/oss-fuzz/pull/9828, where
it looks like we'll have to sort out getting a DEBUG build of LLVM.
