Replace with `execute_process` available on v3.0+

Now saved to variable.

These policies were all introduced before CMake 3.9, so they will automatically
be initialized to the new behavior when requesting a minimum version of 3.9.

Initially I wanted to also do a function that generates the JSON patch from
two JSON documents, but even just applying the JSON patch was a bit of
work, especially when needing to satisfy all the test-cases.

This change defines all the operation in the RFC6902. The addition isn't
too big (for the json_patch_apply() function), as part of the heavy lifting
is also done by JSON pointer logic.

All the ops were tested with the test-cases defined at:
  https://github.com/json-patch/json-patch-tests

RFC6902: https://tools.ietf.org/html/rfc6902

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

That hasn't been needed since since commit 6068d3f, which changed that code to
 check an env var instead ("_JSON_C_STRERROR_ENABLE").
Fixes issue #812, about dup symbols in static builds with clang.

This gets us up to a version that supports features we're already using
(i.e. add_compile_options), but stops short of a cmake that requires
c++11, which some OSes still don't support.
Closes issue #774

Problem: cmake 3.25.1 warns about CMP0042 not being set

Solution: set it explictly to OLD behavior

The warning is as follows:

```
Compatibility with CMake < 2.8.12 will be removed from a future version of CMake.
```

Solution: set it at 2.8.12

Use target_link_libraries, plus fill in Libs.private in json-c.pc so pkg-config --static --libs works appropriately.
Also, only link against libbsd when arc4random is actually found there.

cmake rules matters, and fix compile warnings now that we're building
those sources with all the regular flags.

Mostly found with codespell and during code review.

Update the README to better explain this, and make a few other tweaks.

This change syncs the public header include install location with what gets written into pkgconfig file.

To avoid target exe file export JSON functions.

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>

Fixes the following warning:

json_pointer.c:230:7: warning: implicit declaration of function
    'vasprintf' is invalid in C99 [-Wimplicit-function-declaration]
            rc = vasprintf(&path_copy, path_fmt, args);

Lower overhead than opening & reading from /dev/urandom, and works
in chroots and other situtations where /dev/urandom is not available.
Falls back to existing methods when kernel doesn't support the syscall.