Check if feature macro is defined before define it
Check if feature macro is defined before define it

Zephyr's native posix port define _POSIX_C_SOURCE with a higher value
during the build, so when mbedTLS defines it with a different value
breaks the build.

As Zephyr is already defining a higher value is guaranteed that mbedTLS
required features will be available. So, just define it in case it was
not defined before.

[taken from Zephyr mbedtls module:
https://github.com/zephyrproject-rtos/mbedtls/commit/76dcd6eecae4c1b556a52b1b9ef66e75fc538bc5]

Signed-off-by: Flavio Ceolin <979b9165500b0741b9d0500e2efd74fc1547bff7@intel.com>
Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls