Using thread-local storage may not be desired in all environments
and/or use-cases, thus there should be an option to disable its use
on purpose.

Fixes #451.

Prevent truncation on custom double formatters.

A custom double formatter can lead to truncation of the rest of the
JSON document.

If a custom formatter completely fills the buffer used by snprintf
with a trailing dot or comma and the formatting option
JSON_C_TO_STRING_NOZERO has been specified, then an iterator moves
past the ending '\0' (off-by-one buffer overflow) to set an
additional '\0' and adds the first '\0' into the printbuf.

Since '\0' will eventually be considered the terminating character
of the complete printbuf result, all trailing characters are lost.

This leads to an incomplete JSON string as can be seen with the
test case.

The off-by-one can be noticed if compiled with address sanitizer.

Since this is a very special case and a malformed formatter could
do way more harm and is the responsibility of the user of this
library, this is just a protective measure to keep json-c code as
robust as possible.

Improved support for IBM operating systems

Fix compiler errors and warnings when building on IBM operating systems such as AIX and IBM i.

fix json_parse_uint64() usage of errno

introduced in #542
fixes #601

Add a test to check for this too.

json_parse demo: fix and use usage() function

Support to build both static and shared libraries

Prevent out of boundary write on malicious input

The data structures linkhash and printbuf are limited to 2 GB in size
due to a signed integer being used to track their current size.

If too much data is added, then size variable can overflow, which is
an undefined behaviour in C programming language.

Assuming that a signed int overflow just leads to a negative value,
like it happens on many sytems (Linux i686/amd64 with gcc), then
printbuf is vulnerable to an out of boundary write on 64 bit systems.

If a linkhash with a size of zero is created, then modulo operations
are prone to division by zero operations.

Purely protective measure against bad usage.

If the assignment of stop overflows due to idx and count being
larger than SIZE_T_MAX in sum, out of boundary access could happen.

It takes invalid usage of this function for this to happen, but
I decided to add this check so array_list_del_idx is as safe against
bad usage as the other arraylist functions.

Detect broken RDRAND during initialization

Some CPUs advertise RDRAND in CPUID, but return 0xFFFFFFFF
unconditionally. To avoid locking up later, test RDRAND during
initialization, and if it returns 0xFFFFFFFF, mark it as nonexistent.

Fixes #588.

Fix segmentation fault in CPUID check

Tweak json_parse.c slightly to allow it to build against older json-c versions.

This is intended to provide a way, during development, to test out the memory
and performance impacts of a change.