Check compiletime library version
check_compiletime_version:"1.3.21"

Check runtime library version
check_runtime_version:"1.3.21"

Check for POLARSSL_VERSION_C
check_feature:"POLARSSL_VERSION_C":0

Check for POLARSSL_AES_C when already present
depends_on:POLARSSL_AES_C
check_feature:"POLARSSL_AES_C":0

Check for unknown define
check_feature:"POLARSSL_UNKNOWN":-1