Rename STATIC_LIB macro to ROCKET_STATIC_LIB.

STATIC_LIB name is somewhat general, renaming it to a more rocket specific name avoid conflicts. Especially given the fact that this macro need to be defined in the users code (when linking with the static version of libRocket).

This pull request only affects the main CMakeLists.txt where the definition is appended, various Header.h files, and the mingw build instructions text file.

Drawbacks:
This change is not backward compatible for users that already define STATIC_LIB in their build system, they will need to change it to ROCKET_STATIC_LIB.

Please tell me if you want me to add a preprocessor warning (or an error) message.