Fix TMG: added C function detection when required (e.g. with pastix@6)

Some of the pastix@6 tests require the C interface of TMG library.

Hence, a test on the presence of the C interface for TMG is now required, so that pastix@s6 do not try to compile these tests if this C interface is not found.

Yes it should not have been merged, you should check if the LAPACKE interface has the TMG support. That would be better :)