Refactored RTTI feature detection and moved it to the config header.

[dmitrykos]

In response to #1670 cleans up the fix by moving RTTI detection into the config header.

[coveralls]

Coverage remained the same at 99.625% when pulling 74b2af8 on dmitrykos:cleanup-rtti-detection into 775e760 on cpputest:master.

[dmitrykos]

Had to cleanup PR due to outdated source on my side, now looks fine.

[thetic]

RTTI support can be directly inspected with C++20 https://en.cppreference.com/w/User:D41D8CD98F/feature_testing_macros

[dmitrykos]

@thetic thank you for the proposal, added __cpp_rtti. I did not add a check for its value but checked for presence only similar to GCC's libstdc++.

[basvodde]

Thanks, that looks much better.