fix regression introduced by changeset: 1431:28e9d479aa65 [Use '-p' command line option when running matiec (iec2c)]

Variable matiec_flags in c_ext.py and PythonFileCTNMixin.py is not
used for flags for matiec (IEC to C compiler) actually, but it's used
to compile C-code generated for extensions using normal C-compiler,
like gcc.

                  <xsd:element name="Xenomai">
                    <xsd:complexType>
                      %(toolchain_gcc)s
                      <xsd:attribute name="XenoConfig" type="xsd:string" use="optional" default="xeno-config"/>
                    </xsd:complexType>
                  </xsd:element>