Reduce raw string pointers usage.
* Change some functions to take `std::string` instead of `const char*` in the following classes: `cmMakeFile`, `cmake`, `cmCoreTryCompile`, `cmSystemTools`, `cmState`, `cmLocalGenerator` and a few others. * Greatly reduce using of `const char*` overloads for `cmSystemTools::MakeDirectory` and `cmSystemTools::RelativePath`. * Remove many redundant `c_str()` conversions throughout the code.
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