json-c-0.12.x: Fix CVE-2020-12762 - json-c through 0.14 has an integer overflow and out-of-bounds write ...

This commit is a squashed and slightly modified backport
of the following commits on the master branch:

  * 77d935b
  * d07b910
  * 519dfe1
  * a59d5ac

Json-c 0.12: Fixed Infinity bug

added fallthrough for gcc7

Fix compiler warnings

Bump the version in the release checklist.

* CVE-2013-6371: hash collision denial of service
* CVE-2013-6370: buffer overflow if size_t is larger than int

nan function requires -lm on some platforms - use of NAN is better, if available

Missing lowercase literals test

Only allow lowercase literals in STRICT mode

Avoid potential overflow in json_object_get_double

Conflicts:
	Makefile.am

sscanf is always a potential problem when converting numeric
values as it does not correctly handle over- and underflow
(or at least gives no indication that it has done so).

This change converts json_object_get_double() to use strtod()
according to CERT guidelines.

The -Bsymbolic-functions linker flag reduces the amount of PLT jumps in
a shared object, and has a side effect of preventing symbol collisions
in libraries and applications linking against two different shared
objects exposing the same symbol.

While the former is (generally) a performance win, the latter is less
rare than expected. For instance, PulseAudio started linking against
json-c a while ago; now, every project linking against PulseAudio is
leaking json-c symbols. In the GNOME platform, this means that projects
linking against PulseAudio cannot be safely linked against other
libraries depending on the GLib-based JSON parsing libraries JSON-GLib,
because of a symbol conflict. Nominally, this conflict would not be an
issue: libraries and applications do not need to depend on two different
JSON parsing libraries; the symbol leakage, though, ends up causing
either segmentation faults, or weird errors. For further reference,
please see: https://bugzilla.gnome.org/show_bug.cgi?id=703734

JSON-GLib already switched to using -Bsymbolic-functions, but it would
be safe if json-c did the same, wherever the linker flag is available.

Add json_object_free_userdata() and json_object_userdata_to_json_string() too.