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.

Move http_parser selection into its own cmake module.

There's no need to add the deprecation at the top-level.  Our tests add
deprecation explicitly.

WinHTTP can now be disabled with `USE_WINHTTP=OFF` instead of
`WINHTTP=OFF` to better support the other cmake semantics.

Threading can now be disabled with `USE_THREADS=OFF` instead of
`THREADSAFE=OFF` to better support the other cmake semantics.

Nanosecond support is the default _if_ we can detect it.  This should be
our default always - like threads - and people can opt out explicitly.

path: refactor utility path functions

Support checking for object existence without refresh

Document that `git_odb` is thread-safe

Commit 4ae41f9c made `git_odb`
race-free, and added internal locking. Update `docs/threading.md`
accordingly, so that APIs built atop libgit2 (e.g. language bindings)
can count on this.

Teach `git_path_is_valid` to respect `core.longpaths`.  Add helper
methods to validate length and set the error message appropriately.

Add a `git_str` based validity check; the existing `git_path_is_valid`
defers to it.