This enables ASan, UBSan, et al. and supports both GCC and Clang.

Clang support requires an LLVM-bitcode-parsing-capable linker (because
Clang requires LTO for these sanitizers, and Clang in LTO mode outputs
LLVM bitcode, instead of machine code, leaving it to the linker to
translate it after performing its link-time optimisations).

If you need to, pass LDFLAGS="-fuse-ld=lld" to override the LD variable
set by `./configure` (which isn't used anyway) and use the LLVM linker.

Alternatively, use the Gold linker with the LLVM plugin.

Or just use GCC, but that doesn't support as many sanitizers ...

This commit removes the `--enable-debugging` flag added by commit
447cda49. It wasn't particularly useful anyway. The build
system still checks for CFLAGS="-g", with or without this new option, &
with or without any explicit CFLAGS being passed to `./configure`, so
that the occasionally-submitted backtraces are at least still somewhat
useful.

This commit also makes the CI build script pass the following options
to `./configure`:

    --disable-heap-allocator
    --enable-compiler-sanitizers              (this newly-added option)
    --enable-warnings

The former is so that the sanitizers can catch any memory issues. The
shared heap allocator(s) hide use-after-free problems, because they
don't taint the memory, or release it back to the OS, after Atheme
"frees" it.