Support feature detection with Embarcadero C++ Free compiler

[zedxxx]

It based on Clang compiler: https://www.embarcadero.com/free-tools/ccompiler

Download link: BCC102.zip

[zedxxx]

There are defined macros __BORLANDC__ and __clang__ for this compiler.

[ebiggers]

I haven't used this compiler before, and it doesn't appear to be commonly used. It also doesn't appear to be easily installable on Linux like apt-get install clang (presumably because it is closed source?). So I am not planning to test it myself. If you would like some things to be changed to support it anyway, then please submit a pull request.

[kmilos]

Maybe the mingw patches help with some ideas how to build as clang was recently enabled there?