Three misspellings in comments: "reuired" -> "required",
"ambigous" -> "ambiguous", "neccessarily" -> "necessarily".

Per docs/contributing.md, the amalgamated files under extras/ are left
untouched even though they carry the same typos.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

All the previous refactoring to make the assertion fast paths
smaller and faster also allows us to implement the fast paths
just with thread-local and atomic variables, without full mutexes.

However, the performance overhead of thread-safe assertions is
still significant for single threaded usage:

|  slowdown |  Debug  | Release |
|-----------|--------:|--------:|
| fast path |   1.04x |   1.43x |
| slow path |   1.16x |   1.22x |

Thus, we don't make the assertions thread-safe by default, and instead
provide a build-time configuration option that the users can set to get
thread-safe assertions.

This commit is functional, but it still needs some follow-up work:
 * We do not need full seq_cst increments for the atomic counters,
   and using weaker ones can be faster.
 * We brute-force updating the reporter-friendly totals from internal
   atomic counters by doing it everywhere. We should properly trace
   where this is needed instead.
 * Message macros (`INFO`, `UNSCOPED_INFO`, `CAPTURE`, etc) are not
   made thread safe in this commit, but they can be made thread safe
   in the future, by building on top of this work.
 * Add more tests, including with thread-sanitizer, and compiled
   examples to the repository. Right now, these changes have been
   compiled with tsan manually, but these tests are not added to CI.

Closes #2948

This enables us to generate pkg-config with proper paths if the
installation prefix is decided at install time, e.g. with
```
cmake --install build-dir --prefix /path/to/somewhere
```

It also means that we can use CMake's generator expression to get
the _real_ final name of the libraries, e.g. including debug postfix.

Closes #2979

2 spaces seems to be the more common indentation level so that's what
I unified around.

Turns out that even in GCC, the expression in `__builtin_cosntant_p`
can end up evaluated and side-effects executed. To allow users to
work around this bug, I added a configuration option to disable its
use in internal macros.

Related to #2925

Closes #2854

These options are rather low-level and don't need to be seen in the
CMake cache unless you opt into seeing all other advanced options.

This removes a lot of cache entries from the screen when using a GUI
or TUI to view the cache thus making it easier for users to focus on
the cache variables they're more likely to change on a frequent
basis.