Fix bad CMake argument in README

Introduce compiler check if const is casted away.

There was a long running discussion here
https://github.com/DaveGamble/cJSON/pull/80 how to provide const
correctness for users of cJSON.

To avoid breaking changes for users of cJSON v1 it was decided to
disable this warning.

pragma was tested with gcc 5.4.0/6.2.1 and clang 3.8/3.9.

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.