json_pointer: allow the feature to be disabled

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>

tests/CMakeLists.txt

@@ -24,7 +24,6 @@ set(ALL_TEST_NAMES
     test_double_serializer
     test_float
     test_int_add
-    test_json_pointer
     test_locale
     test_null
     test_parse
@@ -37,6 +36,10 @@ set(ALL_TEST_NAMES
     test_visit
     test_object_iterator)
 
+if (NOT DISABLE_JSON_POINTER)
+    set(ALL_TEST_NAMES ${ALL_TEST_NAMES} test_json_pointer)
+endif()
+
 foreach(TESTNAME ${ALL_TEST_NAMES})
 
 add_executable(${TESTNAME} ${TESTNAME}.c)