Instead of `git__date`, just use `git_date`.

cmake: re-enable WinHTTP

Ensure that we `enable_testing()` at the top-level CMakeLists.txt or
else we'll need to navigate within the build directory to the correct
place in the hierarchy to run `ctest`.  Now we can `ctest` at the
top-level again.

tag: set validity to 0 by default

`git_tag_name_is_valid` needs to set validity to 0 when it
short-circuits.

cmake: further refactorings

The select hashes module selects the hash; the CMakeLists.txt selects
the files to implement it.

Update the global variables `LIBGIT2_OBJECTS` to
`LIBGIT2_DEPENDENCY_OBJECTS` for clarity and consistency.

Apply the standard project cmake formatting to the modules.

When building under gcc 11, there is a warning about an incompatible pointer
type, since
[`__atomic_exchange`](https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html)
does not take `volatile` pointers:

```
In file included from ../src/common.h:81,
                 from ../src/transports/winhttp.c:8:
../src/thread-utils.h: In function ‘git___swap’:
../src/thread-utils.h:168:9: warning: argument 3 of ‘__atomic_exchange’ discards ‘volatile’ qualifier [-Wincompatible-pointer-types]
  168 |         __atomic_exchange(ptr, &newval, &foundval, __ATOMIC_SEQ_CST);
      |         ^~~~~~~~~~~~~~~~~
```

This change drops the `volatile` qualifier so that the pointer type matches
what `__atomic_exchange` expects.

When building under gcc 11, there is a warning about a misaligned guard clause
because there were mixed spaces and tabs:

```
[128/634] Building C object src/CMakeFiles/git2internal.dir/threadstate.c.o
../src/threadstate.c: In function ‘threadstate_dispose’:
../src/threadstate.c:39:5: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
   39 |     if (threadstate->error_t.message != git_str__initstr)
      |     ^~
../src/threadstate.c:41:9: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
   41 |         threadstate->error_t.message = NULL;
      |         ^~~~~~~~~~~
../src/threadstate.c: At top level:
```

This change indents the code with tabs for consistency with the rest of the
code, which makes the warning go away.

Add missing-declarations warning globally

We should enforce declarations throughout the code-base, including
examples, fuzzers and tests, not just in the `src` tree.

Older versions of gcc do not believe that we've adequately declared our
test functions.  Include `clar_suite.h` conditionally for those old
versions.  Do not do this on newer compilers to avoid unnecessary
recompilation of the entire suite when we add or remove a test function.

We may want to have test function declarations; produce a header file
with (only) the test declarations.  Update clar to avoid overwriting the
file unnecessarily to avoid bumping timestamps and potentially
recompiling unnecessarily.

cmake refactorings

str: git_str_free is never a function

Move zlib selection into its own cmake module.

Move WinHTTP selection into its own cmake module.

Move SSH selection into its own cmake module.

Move regex selection into its own cmake module.