Some users may not want to included it in their build/system. So allow a
cmake symbol to disable it.

A user can do 'cmake -DDISABLE_JSON_POINTER=ON <json_c_root_dir>' and
disable the json_pointer functionality. That saves about 17 KB (on an
x86_64) machine. This may be useful on smaller embedded systems; even
though the saving would be fewer kilobytes.

One thing that also needs to change a bit, is that the 'json.h' be
autogenerated via cmake, in order to conditionally include that
"json_pointer.h" file.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

The intent is to be able to disable some features that get built into the
library. When we do that, we also need to disable some tests.

It's easier when adjusting a variable that contains the list of test names,
versus modifying the list in the foreach() statement.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

The function _json_c_strerror does not properly format unknown errnos.
The int to ascii loop ignores the leading digit if the number can be
divided by 10 and if an errno has been formatted, shorter errnos would
not properly terminate the newly created string, showing the ending
numbers of the previous output.

A test case has been added to show these effects.

Since this function has been introduced for tests, the effect of this on
real life code is basically non-existing. First an environment variable
has to be set to activate this strerror code and second an unknown errno
would have to be encountered.

Contrary to other CMAKE variables the CMAKE_C_FLAGS variable is the
composed string of flags for the C compiler. It is therefore not a list
to append to. Current implementation results in these incorrect CFLAGS,
e.g., "-O2 -g -fblahblah;-UNDEBUG". Extending the CFLAGS this way
results in the proper CFLAGS, e.g., "-O2 -g -fblahblah -UNDEBUG".