Hxcpp 4.3.0 has an mbedtls_config.h file in ${HXCPP}/src/hx/libs/ssl,
which we have to ensure is included, but older versions do not.
To support both 4.3.0 and older versions, we can add an empty
mbedtls_config.h and add the include path at the end. This way it will
only be used if it does not exist in the previous include paths.
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