fix memory leak in cJSON_Duplicate

Add a comment that warns about potential write access to constant memory
locations after using cJSON_AddItemToObjectCS.

Make print_preallocated static.

Otherwise compilation failes due to missing prototypes.

gcc check strict overflow v2

This switch is only active when making a release build.

Otherwise, the check is just undefined behaviour. gcc even takes out
this check because len can never be zero if len does not wrap around.

Found with -Wstrict-overflow=2

Enhance gcc checks v2

This switch warns about missing prototypes. It does not warn if
functions are declared static. So this switch encourages the usage of
static whenever possible. This enables the compiler to perform inlining.

This is a prerequisite to later enable -Wmissing-prototypes.

CMake: Option was not enabled by default as intended.
Makefile: Flags were not passed when building the tests

This is not changing the behavior of the code, but it is improving the
readability and helps to make the intention clearer.

One thing to take care: 'true' should never be compared against.