"sphinx-build" linkcheck fails too often to be worthwhile, due GitHub
outages and other problems.

Since 933fd981, the JSON reader emitted an
error when encountering an integer value for a $date key. This restores the
previous behavior and allows legacy $date syntax to be parsed properly.
Consequently, a BSON corpus parse error test must now be skipped.

This commit adds previously absent tests for legacy $date syntax and adds
more valid and error cases to the existing $date $numberLong test.

Since d7406f5d, all parse error tests were
executing with bson_new_from_json(), which is only appropriate for type 0x00.

This commit also restructures the corpus test runner so that callbacks may be
provided for decode and parse error tests as they are for valid tests. This
change will make it easier to skip tests of any type, which will be necessary
for CDRIVER-2223.

In the future we may need to check whether static dependencies are
available. For now, AUTO means ON.

Introduce new functions for the canonical and relaxed formats from Extended
JSON Spec 2.0. This reverts previous changes to bson_as_json(), which might
have caused BC breaks. Maintain legacy bson_as_json() output for non-finite
values. Note that legacy mode will produce "nan" and "inf" literals, which
are not valid JSON.

Also adds helper function for appending sorted regex options and regression
tests for bson_as_json() and multiple types.

This causes warnings on newer GCC and Clang versions that enable
-Wexpansion-to-defined by default.

Error code reporting in .bat files is broken in Windows.

For backwards compatibility, defaults to ON. You can turn it OFF to skip
building and installing static libbson. ENABLE_TESTS must also be OFF.

Ensure that the regex option flags "ilmsux" are stored in sorted order,
even when passed to bson_append_regex.

By the time JSONSL checks if a u-escape value is less than 0xDC00, it
already knows it is greater than 0xD7FF.

A change in the Extended JSON Spec.

Extended JSON spec prohibits it, only {"$date": "2017-06-07T12:34:45"}
or {"$date": {"$numberLong": "12345"}} are allowed.