lhapdf: disable $(srcdir) in TESTS

This is not supported by autotools. Only affects LHAPDF version 5.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
And also this one is probably better than my fix (which removes octave completely). However I'll implement it in the cms-externals/lhapdf repository rather than as a patch.