Hxcpp's mbedtls has MBEDTLS_NET_C disabled, which meant that older
versions of curl which use this feature cannot be linked against it.
This version of curl no longer requires this feature, which avoids the
issue.
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.
