Support hxcpp older than 4.3.0 for mbedtls linking

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.

project/lib/curl-files.xml

@@ -30,6 +30,7 @@
 
 			<compilerflag value="-I${MBEDTLS_DIR}/include" />
 			<compilerflag value="-I${HXCPP}/src/hx/libs/ssl" />
+			<compilerflag value="-I${this_dir}/custom/mbedtls_hxcpp" />
 			<compilerflag value="-DMBEDTLS_USER_CONFIG_FILE=&lt;mbedtls_config.h&gt;" />
 
 		</section>