Fix various potential null ptr deref and int32 overflows

Add utility function for comparing json_objects

Fix issue #221: JSON_C_TO_STRING_NOSLASHESCAPE works incorrectly

Clarify json_object_get_string documentation of NULL handling & return

Tests added.

Do some basic checks on ints, doubles, strings, arrays and "complex" objects.

This fix errors that can happen when ingesting very large JSON files
when hitting the maximum heap size of the process.

The [AC_PROG_CC_C99 macro](https://www.gnu.org/software/autoconf/manual/autoconf-2.64/html_node/C-Compiler.html) sets the complier to the C99 standard if the compiler does not default to such behaviour (albeit gnu99). This is better than specifically hardcoding flags

AC_FUNC_REALLOC is messed up when cross-compiling, ending up in failed
build with "undefined reference to `rpl_realloc'" error.
AC_CHECK_FUNCS will work both with native and cross builds.

fix regression from 2d549662be

Fix possible memory leak and remove superfluous NULL checks before free()

That commit introduced read-only keys, but when the hash table
was resized, that attribute was not preserved. This resulted in
an invalid free at time of table destruction.

Fix build in Visual Studio

Add build scripts for CI platforms

Performance improvements

smalls strings inside json_objects had a high overhead because dynamic
memory allocation was needed for each of them. This also meant that the
pointer needed to be updated. This is now changed so that small strings
can directly be stored inside the json_object. Note that on the regular
64 bit machines a pointer takes 8 bytes. So even without increasing
memory, we could store string up to 7 bytes directly inside the object.
The max size is configurable. I have selected up to 31 bytes (which
means a buffer of 32 including the NUL byte). This brings a 24-bytes
memory overhead, but I consider that still useful because the memory
allocator usually also has quite some overhead (16 bytes) for
dyn alloced memory blocks. In any case, the max buffer size can be
tweaked via #define.

These items were used for statistics tracking, but no code at all
exists to consume them. By removing them we save

a) space
   because they counters required space, and did so in each and every
   json object

b) performance
   because calloc() needs to write less data and the counters are
   no longer maintained; cache performance can be better, load
   on OS main memory is lighter

We could conditionally enable/disable these counters, but I have not
done this they were really nowhere used and it looked more like a
left-over from the import of hashtable code.

This also adds a new API json_global_set_string_hash() which permits
to select the hash function. The default one is the only one that was
previously present. So there are no changes to existing apps, and the
new hash function needs to be explicitely be opted in. Especially for
smaller strings, the perllike functions seems to be around twice as
fast as the other one, with similarly good results in value distribution.

This provides more control over some detail aspects, many
of which are performance related.

This can be a very considerable performance saver.

It's needed to squash a "variable 'val' set but not used" warning.

Apply compile warning fix to master branch

Syntax error

- ./configure would fail due to bad code generation